---
title: "Organic Molecules: Skeletons and Names"
book: "School Chemistry — Grades 1 to 12"
subject: chemistry
language: en
chapter: 32
exercises: 15
source: https://one-course.com/books/chemistry/1/en/chapter/32-organic-molecules-skeletons-and-names
license: CC-BY-NC-SA-4.0
credit: "One Chemistry Book, One Course (one-course.com)"
---

# Chapter 32 — Organic Molecules: Skeletons and Names

A disposable lighter holds butane, liquid under pressure; the cartridge of a camping stove taken up a snowy mountain holds propane, or a blend richer in it. Both are made of nothing but carbon and hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom), and both belong to a huge family: the [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) built on a skeleton of carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom). Millions of them are known. To talk about them, chemists need ways of drawing them quickly and of naming them without ambiguity.

**You already know.**

Carbon forms four [covalent bonds](https://one-course.com/books/chemistry/1/en/chapter/24-lewis-structures-and-the-shape-of-molecules#def-g10-lewis-and-shape-covalent-bond) and hydrogen one; around a carbon [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) with four single bonds, the bonds point to the corners of a tetrahedron, at about $109.5{}^{\circ}$ ([Chapter 24](https://one-course.com/books/chemistry/1/en/chapter/24-lewis-structures-and-the-shape-of-molecules#ch-g10-lewis-and-shape)).

![A camping stove in the snow, burning a gas made of carbon and hydrogen.](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/img-87e2deda58fb.jpg)

*A camping stove in the snow, [burning](https://one-course.com/books/chemistry/1/en/chapter/5-burning-what-a-fire-needs#def-g4-what-a-fire-needs-burning) a gas made of carbon and hydrogen.*

## 32.1 Formulas of organic molecules

**Definition 32.1 (Organic compound).**

An *organic compound* is a compound whose [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) are built on a skeleton of carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) bonded to one another and to hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom), possibly with other [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) such as oxygen and nitrogen. (Carbon dioxide, carbon monoxide and the carbonates are traditionally left out.)

**Definition 32.2 (Four kinds of formula).**

A [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) can be written in four ways:

- its *molecular formula* gives only the number of [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) of each element, such as $\ce{C4H10}$ ;
- its *structural formula* shows every [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) and every bond;
- its *semi-structural formula* shows the bonds between carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) and groups the hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) with their carbon, such as $\ce{CH3-CH2-CH2-CH3}$ ;
- its *skeletal formula* draws only the bonds of the carbon skeleton as a zigzag: each end and each corner is a carbon [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) , the hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) on carbon are not drawn, and every other [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) (with its hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) ) is written.

![The four formulas of butane and of 2-methylpropane. In the skeletal formulas, each end and each corner of the lines is a carbon atom: four in each.](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-c6fb89b46f2a.svg)

*The four formulas of butane and of 2-methylpropane. In the [skeletal formulas](#def-g11-organic-skeletons-formulas), each end and each corner of the lines is a carbon [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom): four in each.*

**Example 32.3 (Reading a skeletal formula).**

In the [skeletal formula](#def-g11-organic-skeletons-formulas) of butane, the zigzag has two ends and two corners: four carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom). Each end carbon has one bond drawn, so it carries three hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) ($\ce{CH3}$); each corner has two bonds drawn, so it carries two ($\ce{CH2}$). Every carbon has four bonds in all, which gives back $\ce{C4H10}$.

## 32.2 Carbon chains

The carbon skeleton of a [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) can be a *linear* chain, each carbon bonded to at most two others, as in butane; a *branched* chain, with at least one carbon bonded to three or four others, as in 2-methylpropane; or a *ring*, as in cyclohexane, $\ce{C6H12}$, whose [skeletal formula](#def-g11-organic-skeletons-formulas) is a hexagon. A zigzag is used for chains because the real bonds around each carbon make angles of about $109.5{}^{\circ}$, not $180{}^{\circ}$: a “linear” chain is in fact a zigzag in space.

![Three carbon skeletons, in skeletal formulas.](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-9142de51d0d7.svg)

*Three carbon skeletons, in [skeletal formulas](#def-g11-organic-skeletons-formulas).*

## 32.3 Alkanes and their names

**Definition 32.4 (Alkane, alkyl group).**

An *alkane* is a compound of carbon and hydrogen whose skeleton is a chain (linear or branched, not a ring) with only single bonds; its [molecular formula](#def-g11-organic-skeletons-formulas) is $\ce{C_{n}H_{2n+2}}$. An *alkyl group* is what remains of an alkane when one hydrogen [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) is removed, so that it can be attached to a chain: methyl $\ce{-CH3}$, ethyl $\ce{-CH2-CH3}$.

| $n$ | name | formula | $n$ | name | formula |
| --- | --- | --- | --- | --- | --- |
| 1 | methane | $\ce{CH4}$ | 6 | hexane | $\ce{C6H14}$ |
| 2 | ethane | $\ce{C2H6}$ | 7 | heptane | $\ce{C7H16}$ |
| 3 | propane | $\ce{C3H8}$ | 8 | octane | $\ce{C8H18}$ |
| 4 | butane | $\ce{C4H10}$ | 9 | nonane | $\ce{C9H20}$ |
| 5 | pentane | $\ce{C5H12}$ | 10 | decane | $\ce{C10H22}$ |

*The first ten linear [alkanes](#def-g11-organic-skeletons-alkane). The name of a chain of $n$ carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) is the same root followed by *-ane*; the [alkyl group](#def-g11-organic-skeletons-alkane) replaces *-ane* by *-yl* (methyl, ethyl, propyl…).*

**Method 32.5 (Naming an alkane).**

1. Find the longest chain of carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) : it gives the root name (pentane, hexane…).
2. The [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) not in it form [alkyl groups](#def-g11-organic-skeletons-alkane) attached to it, the substituents.
3. Number the main chain from the end that gives the substituents the lowest numbers.
4. Write each substituent with its number (its position on the chain), in alphabetical order, before the root; repeated groups take the prefixes di-, tri-, tetra-, with one number each. Numbers are separated by commas, numbers and words by hyphens.

![Naming a branched alkane: the main chain (numbered), the substituents (circled), the name.](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-68fad2f41f0d.svg)

*Naming a branched [alkane](#def-g11-organic-skeletons-alkane): the main chain (numbered), the substituents (circled), the name.*

**Method 32.6 (From a name to a skeletal formula).**

1. Draw the main chain given by the root, as a zigzag, and number its carbons.
2. Attach each substituent at the position given by its number.
3. Check: count the carbons, and that no carbon has more than four bonds.

## 32.4 Isomers

**Definition 32.7 (Isomers, constitutional isomers).**

*Isomers* are different compounds with the same [molecular formula](#def-g11-organic-skeletons-formulas). *Constitutional isomers* are isomers whose [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) are bonded in a different order, for example with a different carbon skeleton.

**Proposition 32.8 (Isomers have different properties).**

[Constitutional isomers](#def-g11-organic-skeletons-isomer) are different substances, with different properties. Among the [alkane](#def-g11-organic-skeletons-alkane) [isomers](#def-g11-organic-skeletons-isomer) of a given formula, the more branched ones boil lower: butane boils near $0\,{}^{\circ}\mathrm{C}$ and 2-methylpropane at $-11.7\,{}^{\circ}\mathrm{C}$; pentane at $36.1\,{}^{\circ}\mathrm{C}$, 2-methylbutane at $27.8\,{}^{\circ}\mathrm{C}$ and 2,2-dimethylpropane at $9.5\,{}^{\circ}\mathrm{C}$.

**Proof.** The values are measured. Branched [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) are more compact, with less surface in contact with their neighbours: their [van der Waals interactions](https://one-course.com/books/chemistry/1/en/chapter/30-electronegativity-polarity-and-intermolecular-forces#def-g11-polarity-and-cohesion-van-der-waals) are weaker ([Chapter 30](https://one-course.com/books/chemistry/1/en/chapter/30-electronegativity-polarity-and-intermolecular-forces#ch-g11-polarity-and-cohesion)). ∎

![The three isomers of C5H12 and their boiling temperatures: the more branched, the lower.](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-f5c89c933481.svg)

*The three [isomers](#def-g11-organic-skeletons-isomer) of $\ce{C5H12}$ and their boiling temperatures: the more branched, the lower.*

## 32.5 Alkanes from crude oil

Crude oil is a [mixture](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-mixture) of thousands of compounds, most of them [alkanes](#def-g11-organic-skeletons-alkane) with 1 to more than 40 carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom). In a refinery it is heated and its vapours rise through a tall column, cooler and cooler towards the top: each fraction condenses at the height where the temperature matches its boiling range. Gases (methane to butane) leave at the top, then petrol, kerosene, diesel; heavy oils and bitumen stay at the bottom. This fractional [distillation](https://one-course.com/books/chemistry/1/en/chapter/22-chemical-species-natural-and-synthetic#def-g10-chemical-species-distillation) separates by boiling temperature, which for [alkanes](#def-g11-organic-skeletons-alkane) grows with the length of the chain.

![Distillation columns of a refinery at dusk.](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/img-8bb78c1bfb9a.jpg)

*[Distillation](https://one-course.com/books/chemistry/1/en/chapter/22-chemical-species-natural-and-synthetic#def-g10-chemical-species-distillation) columns of a refinery at dusk.*

**Safety.**

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-c3411fc37f8b.svg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-35a2a9794e22.svg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-ac31e46e7418.svg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-289d438dca56.svg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-fde1dd431b06.svg)

Butane is an extremely flammable gas, stored under pressure. Pentane is a highly flammable liquid; its vapour causes drowsiness, it can damage the lungs if swallowed and it is toxic to aquatic life. No flame near light [alkanes](#def-g11-organic-skeletons-alkane); work under a fume hood.

## 32.6 Exercises

**Exercise 32.1 ★.**

Give the [molecular formulas](#def-g11-organic-skeletons-formulas) of the [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) drawn here, and say whether they are [isomers](#def-g11-organic-skeletons-isomer): ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-329c3347bcb7.svg) and ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-6b59d51cb000.svg).

**Solution of Exercise 32.1.**

Both have six carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) (ends and corners) and $\ce{C6H14}$: hexane and 2-methylpentane are [isomers](#def-g11-organic-skeletons-isomer).

**Exercise 32.2 ★.**

Name: $\ce{CH3-CH2-CH3}$; $\ce{CH3-CH2-CH2-CH2-CH2-CH3}$; ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-2871b80734ff.svg).

**Solution of Exercise 32.2.**

Propane; hexane; 2-methylbutane.

**Exercise 32.3 ★.**

Write the [semi-structural formulas](#def-g11-organic-skeletons-formulas) of propane and of 2-methylpropane, and the [structural formula](#def-g11-organic-skeletons-formulas) of ethane.

**Solution of Exercise 32.3.**

$\ce{CH3-CH2-CH3}$; ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-750a6992da98.svg); ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-54ec65fe6c6a.svg).

**Exercise 32.4 ★.**

Count the carbon and hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) of each [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule): ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-00f8a2afd78a.svg) and ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-2eb15bc4fdb4.svg).

**Solution of Exercise 32.4.**

The first, 3-methylpentane: 6 carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom); its $\ce{CH3}$ ends (three of them) carry 9 hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom), its two $\ce{CH2}$ corners 4 and its $\ce{CH}$ branch point 1: $\ce{C6H14}$. Cyclohexane: 6 corners, each $\ce{CH2}$: $\ce{C6H12}$.

**Exercise 32.5 ★.**

Give the [molecular formula](#def-g11-organic-skeletons-formulas) of the [alkane](#def-g11-organic-skeletons-alkane) with 8 carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom), and the number of carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) of the [alkane](#def-g11-organic-skeletons-alkane) with 20 hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom).

**Solution of Exercise 32.5.**

$\ce{C8H18}$. $2n + 2 = 20$ gives $n = 9$: nonane, $\ce{C9H20}$.

**Exercise 32.6 ★★.**

Draw the [skeletal formulas](#def-g11-organic-skeletons-formulas) of 3-methylhexane, 2,2-dimethylbutane and 3-ethylpentane, and give their [molecular formulas](#def-g11-organic-skeletons-formulas).

**Solution of Exercise 32.6.**

3-methylhexane ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-a9107f42608b.svg), $\ce{C7H16}$; 2,2-dimethylbutane ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-7cd22edfe135.svg), $\ce{C6H14}$; 3-ethylpentane ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-76e2e94527fb.svg), $\ce{C7H16}$.

**Exercise 32.7 ★★.**

Draw and name the three [isomers](#def-g11-organic-skeletons-isomer) of $\ce{C5H12}$.

**Solution of Exercise 32.7.**

Pentane, 2-methylbutane and 2,2-dimethylpropane, drawn in the figure of the chapter.

**Exercise 32.8 ★★.**

These names are wrong. Draw each [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) and give its correct name: 2-ethylbutane; 4-methylpentane; 1-methylbutane.

**Solution of Exercise 32.8.**

“2-ethylbutane” has a longest chain of five carbons through the ethyl group: 3-methylpentane. “4-methylpentane” is numbered from the wrong end: 2-methylpentane. “1-methylbutane” is simply a chain of five carbons: pentane.

**Exercise 32.9 ★★.**

Using the figure of the [isomers](#def-g11-organic-skeletons-isomer) of $\ce{C5H12}$, rank them by boiling temperature and explain the order.

**Solution of Exercise 32.9.**

2,2-dimethylpropane ($9.5\,{}^{\circ}\mathrm{C}$) < 2-methylbutane ($27.8\,{}^{\circ}\mathrm{C}$) < pentane ($36.1\,{}^{\circ}\mathrm{C}$). The same [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) are arranged more and more compactly: less contact between [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule), weaker [van der Waals interactions](https://one-course.com/books/chemistry/1/en/chapter/30-electronegativity-polarity-and-intermolecular-forces#def-g11-polarity-and-cohesion-van-der-waals), lower boiling temperature.

**Exercise 32.10 ★★.**

Cyclohexane is $\ce{C6H12}$ and hexane $\ce{C6H14}$. Explain the two missing hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom). Are the two compounds [isomers](#def-g11-organic-skeletons-isomer)? Is cyclohexane an [alkane](#def-g11-organic-skeletons-alkane)?

**Solution of Exercise 32.10.**

Closing the chain into a ring makes one more $\ce{C-C}$ bond, which takes the place of two $\ce{C-H}$ bonds (one at each end). Different [molecular formulas](#def-g11-organic-skeletons-formulas): not [isomers](#def-g11-organic-skeletons-isomer). Cyclohexane is not an [alkane](#def-g11-organic-skeletons-alkane) (its skeleton is a ring; its formula is not $\ce{C_{n}H_{2n+2}}$).

**Exercise 32.11 ★★.**

Among propane, butane, 2-methylpropane and cyclobutane ($\ce{C4H8}$, a ring of four carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom)), which are [isomers](#def-g11-organic-skeletons-isomer) of one another?

**Solution of Exercise 32.11.**

Butane and 2-methylpropane ($\ce{C4H10}$). Propane is $\ce{C3H8}$ and cyclobutane $\ce{C4H8}$.

**Exercise 32.12 ★★★.**

Draw and name the five [isomers](#def-g11-organic-skeletons-isomer) of $\ce{C6H14}$.

**Solution of Exercise 32.12.**

| ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-88aeee31d81b.svg) | ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-d49c758c8333.svg) | ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-c491fdf6fadc.svg) |
| --- | --- | --- |
| hexane | 2-methylpentane | 3-methylpentane |
| ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-fc4504ea648b.svg) | ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-ce8fba9907d7.svg) |  |
| 2,2-dimethylbutane | 2,3-dimethylbutane |  |

**Exercise 32.13 ★★★.**

Picture a linear [alkane](#def-g11-organic-skeletons-alkane) as a long zigzag and a very branched one as a ball. Using this picture, explain why branching lowers the boiling temperature of [isomers](#def-g11-organic-skeletons-isomer).

**Solution of Exercise 32.13.**

Two long zigzags can lie side by side along their whole length, with many [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) in contact; two balls touch at only a small area. [Van der Waals interactions](https://one-course.com/books/chemistry/1/en/chapter/30-electronegativity-polarity-and-intermolecular-forces#def-g11-polarity-and-cohesion-van-der-waals) act between [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) in contact: they are larger between linear [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule), which need a higher temperature to separate.

**Exercise 32.14 ★★★.**

Name the [alkane](#def-g11-organic-skeletons-alkane)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-2a371fc8cbed.svg)

**Solution of Exercise 32.14.**

The longest chain has six carbons; numbered from the left the substituents are at 2, 4, 4, from the right at 3, 3, 5; the first difference (2 against 3) decides: 2,4,4-trimethylhexane.

**Exercise 32.15 ★★★.**

The reference compound for petrol, “isooctane”, is 2,2,4-trimethylpentane. Draw its [skeletal formula](#def-g11-organic-skeletons-formulas), check that it is an isomer of octane, and write its [semi-structural formula](#def-g11-organic-skeletons-formulas).

**Solution of Exercise 32.15.**

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-bf10bd033b51.svg): a chain of five carbons with two methyl groups on carbon 2 and one on carbon 4, eight carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) in all; $2 \times 8 + 2 = 18$ hydrogen [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom), $\ce{C8H18}$, the formula of octane: they are [isomers](#def-g11-organic-skeletons-isomer). [Semi-structural formula](#def-g11-organic-skeletons-formulas): ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-0fab45f71dbf.svg).

## 32.7 Problem: Lighter Gas

**Problem 32.1.**

Weekend problem — how many different [alkanes](#def-g11-organic-skeletons-alkane) have the formula $\ce{C7H16}$?

A lighter holds butane; a winter camping cartridge holds a blend richer in propane or in 2-methylpropane. Butane boils at about $0\,{}^{\circ}\mathrm{C}$, 2-methylpropane at $-11.7\,{}^{\circ}\mathrm{C}$ and propane at $-42\,{}^{\circ}\mathrm{C}$.

**Part I — Two gases with one formula.**

1. Give the [molecular formula](#def-g11-organic-skeletons-formulas) of butane, and check that it fits the general formula of the [alkanes](#def-g11-organic-skeletons-alkane) .
2. Draw its [structural formula](#def-g11-organic-skeletons-formulas) .
3. Write its semi-structural and [skeletal formulas](#def-g11-organic-skeletons-formulas) .
4. Draw 2-methylpropane in the same three ways. Why does it have the same [molecular formula](#def-g11-organic-skeletons-formulas) ?
5. Are butane and 2-methylpropane [isomers](#def-g11-organic-skeletons-isomer) ? Of which kind?

**Part II — In the cold.**

6. Which of the three gases are gases at $20\,{}^{\circ}\mathrm{C}$ and normal pressure?
7. An open lighter is used outdoors at $-5\,{}^{\circ}\mathrm{C}$ . Why does it give a poor flame?
8. Why are winter cartridges rich in propane or in 2-methylpropane?
9. Explain why 2-methylpropane boils lower than butane.
10. Why does propane have no isomer?

**Part III — Naming and counting.**

11. Name ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-da06593f7fee.svg) .
12. Why is the name “3-methylbutane” wrong for the same [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) ?
13. How many [alkane](#def-g11-organic-skeletons-alkane) [isomers](#def-g11-organic-skeletons-isomer) have the formulas $\ce{C4H10}$ , $\ce{C5H12}$ and $\ce{C6H14}$ ?

**Part IV — The [isomers](#def-g11-organic-skeletons-isomer) of $\ce{C7H16}$.**

14. Draw the linear isomer and name it.
15. Draw and name the [isomers](#def-g11-organic-skeletons-isomer) whose longest chain has six carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) . Why is there no “4-methylhexane”?
16. Draw and name the [isomers](#def-g11-organic-skeletons-isomer) whose longest chain has five carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) and two methyl groups.
17. Is there an isomer with a five-carbon chain and an ethyl group? Why can the ethyl group only sit on carbon 3?
18. Draw and name the isomer whose longest chain has four carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) .
19. Count all the [isomers](#def-g11-organic-skeletons-isomer) found.
20. State the final answer: how many [constitutional isomers](#def-g11-organic-skeletons-isomer) does heptane have, itself included?

**Solution of Problem 32.1.**

**1.** $\ce{C4H10}$: with $n = 4$, $2n + 2 = 10$.

**2.** ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-c53772dd2ba7.svg).

**3.** $\ce{CH3-CH2-CH2-CH3}$; ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-922a7cdf08c5.svg).

**4.** ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-43a1f07776d2.svg); ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-116e9ce3f2ab.svg); ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-678ff4f833fc.svg). It has the same [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom), four carbons and ten hydrogens, bonded in a different order.

**5.** Yes: [constitutional isomers](#def-g11-organic-skeletons-isomer) (different skeletons).

**6.** All three: their boiling temperatures are below $20\,{}^{\circ}\mathrm{C}$.

**7.** At $-5\,{}^{\circ}\mathrm{C}$, below its boiling temperature, butane stays liquid at normal pressure and gives off little gas.

**8.** Propane ($-42\,{}^{\circ}\mathrm{C}$) and 2-methylpropane ($-11.7\,{}^{\circ}\mathrm{C}$) boil below winter temperatures: they still give plenty of gas in the cold.

**9.** 2-methylpropane is branched, more compact: weaker [van der Waals interactions](https://one-course.com/books/chemistry/1/en/chapter/30-electronegativity-polarity-and-intermolecular-forces#def-g11-polarity-and-cohesion-van-der-waals).

**10.** With three carbon [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) the only possible skeleton is the chain $\ce{C-C-C}$: any carbon moved elsewhere gives the same chain.

**11.** 2-methylbutane.

**12.** The chain must be numbered from the end nearer the substituent: 2, not 3.

**13.** 2, 3 and 5.

**14.** Heptane, ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-def0a4d91494.svg).

**15.** 2-methylhexane ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-89148cf10297.svg) and 3-methylhexane ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-d01fe9df0c22.svg). “4-methylhexane” is 3-methylhexane numbered from the wrong end.

**16.** 2,2-dimethylpentane, 2,3-dimethylpentane, 2,4-dimethylpentane and 3,3-dimethylpentane.

**17.** Yes, 3-ethylpentane. On carbon 2 the ethyl group would make a six-carbon chain through it: the [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) would be 3-methylhexane.

**18.** 2,2,3-trimethylbutane, ![](https://one-course.com/images/onecourse/chapters/chemistry-1/g11-organic-skeletons/fig-ba64feb380c4.svg).

**19.** $1 + 2 + 4 + 1 + 1 = 9$.

**20.** Heptane has 9 [constitutional isomers](#def-g11-organic-skeletons-isomer), itself included.