Exercises: Hydrocarbons

The natural gas supplied to kitchens is mostly methane, and alkanes are the simplest family of hydrocarbons.

1. a) Write the molecular formulas of methane, ethane and propane.
2. b) What type of bond — single, double or triple — joins the carbon atoms together in alkanes?
3. c) Explain what the '-ane' ending in their names indicates.

In body shops, welding is done using ethyne (acetylene), and fruit releases ethene as it ripens.

1. a) Using the general formulas for alkenes, $\text{C}_n\text{H}_{2n}$, and alkynes, $\text{C}_n\text{H}_{2n-2}$, with $n = 2$, write the molecular formula for each one.
2. b) State what type of bond joins their carbon atoms.
3. c) State which family each belongs to and which IUPAC ending corresponds to it.

Petrol contains medium-chain alkanes.

1. a) Write the molecular formulas of pentane, hexane, heptane and octane.
2. b) By observing how the subscripts change, deduce the general formula for alkanes in terms of the number of carbons $n$.
3. c) Check that your general formula gives the correct result for octane.

Structural formulas group each carbon with its hydrogens and show the carbon chain.

1. a) Write the structural formula of propane and that of butane.
2. b) Also write their molecular formulas.
3. c) Count the total number of atoms in a butane molecule.

The prefixes in organic nomenclature indicate how many carbons are in the main chain.

1. a) How many carbons do the prefixes meth-, eth-, prop-, but-, pent- and hex- indicate?
2. b) The endings -ane, -ene and -yne give information about the type of bond between the carbons: explain what each one indicates.
3. c) Is the name 'butene', on its own, complete? Reason what information is missing.

Classify these three hydrocarbons using the general formulas for alkanes, $\text{C}_n\text{H}_{2n+2}$, alkenes, $\text{C}_n\text{H}_{2n}$, and alkynes, $\text{C}_n\text{H}_{2n-2}$, and name each one:

1. a) $\text{C}_3\text{H}_8$
2. b) $\text{C}_3\text{H}_6$
3. c) $\text{C}_3\text{H}_4$

Benzene, $\text{C}_6\text{H}_6$, is the best-known aromatic compound: today it is only used as an industrial raw material (plastics such as polystyrene) because it is toxic and carcinogenic. Its ring with delocalised electrons is shown in the figure.

1. a) How many carbon atoms form its ring?
2. b) The six carbon–carbon bonds in the ring are all equal, intermediate between a single and a double bond. Explain in your own words what it means for its electrons to be 'delocalised' throughout the ring.
3. c) Why do you think its use as a solvent is prohibited in school laboratories?

Cyclohexane is an industrial solvent whose molecule is a ring of six $\text{CH}_2$ groups. The figure shows this ring.

1. a) Write its molecular formula.
2. b) Compare it with that of hexane, $\text{C}_6\text{H}_{14}$, and explain why cyclohexane has two fewer hydrogens.
3. c) Which family does cyclohexane belong to?

2-methylbutane, a branched component of petrol, has the structural formula $\text{CH}_3\text{-CH(CH}_3\text{)-CH}_2\text{-CH}_3$. The figure shows its carbon skeleton.

1. a) How many carbons does the main chain have, and why is that particular chain chosen?
2. b) What position does the methyl group occupy, and how is this indicated in the name?
3. c) Write its molecular formula and check that it satisfies the general formula for alkanes.

Give the structural formulas for the following branched alkanes, commonly used as solvents and in fuel blends:

1. a) 3-methylpentane.
2. b) 2,2-dimethylpropane.
3. c) How many carbons does each one have, and what is its molecular formula?

Pent-2-ene is obtained in a refinery as a raw material for other processes.

1. a) Write its structural formula.
2. b) Explain what the number 2 in its name indicates.
3. c) Write the structural formula of pent-1-ene and state how both compounds differ.

Name the following unsaturated hydrocarbons:

1. a) $\text{CH}_3\text{-CH=CH-CH}_3$
2. b) $\text{CH}\equiv\text{C-CH}_2\text{-CH}_3$
3. c) $\text{CH}_2\text{=CH-CH}_3$

In part c), explain why there is no need to write a number to indicate the position of the double bond.

But-1-ene and but-2-ene are two different substances with the same molecular formula, $\text{C}_4\text{H}_8$.

1. a) Write the structural formulas of both.
2. b) Draw their displayed formulas in your notebook.
3. c) They are called position isomers: explain what this means.

You can do it on paper and send a photo with your answer.

A student claims that the compound $\text{CH}_3\text{-CH}_2\text{-CH(CH}_3\text{)-CH}_2\text{-CH}_3$ is called 2-ethylbutane.

1. a) Draw the chain in your notebook and explain what mistake was made when choosing the main chain.
2. b) Write the correct name.
3. c) State the rule that must be applied to choose the main chain.

You can do it on paper and send a photo with your answer.

Cyclic hydrocarbons can also have substituents: in the paint industry, methylcyclohexane is used as a solvent.

1. a) Its molecule is a cyclohexane ring in which a hydrogen has been replaced by a methyl group: write its condensed structural formula and deduce its molecular formula.
2. b) Write the name of the compound formed by a cyclopentane ring with a methyl group attached to it.
3. c) Check that both satisfy the general formula for cycloalkanes, $\text{C}_n\text{H}_{2n}$.

The molecular formula $\text{C}_6\text{H}_{12}$ can correspond to a hexene or to cyclohexane.

1. a) Write the structural formula of hex-1-ene.
2. b) Draw the cyclohexane ring in your notebook.
3. c) Explain why both share the same molecular formula even though one is an open chain and the other is a ring.

You can do it on paper and send a photo with your answer.

Pentane has three isomers with formula $\text{C}_5\text{H}_{12}$.

1. a) Write the structural formulas of all three and name them.
2. b) Draw their carbon chains in your notebook.
3. c) Their boiling points are $36\ \celsius$, $28\ \celsius$ and $9,5\ \celsius$. Assign each value to an isomer and justify the trend: the more branching there is, the less surface area of contact between molecules and the easier it boils.

You can do it on paper and send a photo with your answer.

A hydrocarbon has the structural formula $\text{CH}_3\text{-CH(CH}_3\text{)-CH}_2\text{-CH(C}_2\text{H}_5\text{)-CH}_3$.

1. a) Draw it in your notebook and find the longest continuous chain: how many carbons does it have? (Note: it is not the one written horizontally.)
2. b) Number the chosen chain from the end that gives the lowest locants.
3. c) Write its full IUPAC name.

When a molecule has branches and multiple bonds, numbering gives priority to the double or triple bond.

1. a) Name $\text{CH}_2\text{=CH-CH(CH}_3\text{)-CH}_2\text{-CH}_3$.
2. b) Write the structural formula of 4-methylpent-2-yne.
3. c) State the rule you applied to number the chain in both cases.

In 1865, Kekulé proposed that benzene was a ring with three alternating double bonds, but experiments show that all six carbon–carbon bonds are exactly the same length, an intermediate length between a single and a double bond.

1. a) Draw the Kekulé structure in your notebook and, alongside it, the modern model with a circle inside the ring.
2. b) Explain what this circle means: where are the electrons of the 'double bonds'?
3. c) Cyclohexene decolorises bromine water (a typical reaction of double bonds) whereas benzene does not. Use the delocalised electron model to justify why benzene does not behave like an alkene.

You can do it on paper and send a photo with your answer.

The octane rating of a petrol compares its behaviour with that of 2,2,4-trimethylpentane, which is assigned a value of 100.

1. a) Write its structural formula. (It is helpful to write the 5-carbon chain first and then add the three methyl groups.)
2. b) Deduce its molecular formula and check that it is an isomer of octane, $\text{C}_8\text{H}_{18}$.
3. c) Reason, from its name, how much branching it has and compare its structure with that of straight-chain octane.

The analysis of an unknown hydrocarbon results in the molecular formula $\text{C}_7\text{H}_{12}$.

1. a) Which families of hydrocarbons satisfy the general formula $\text{C}_n\text{H}_{2n-2}$? Mention two.
2. b) Write the structural formula of a possible compound with 7 carbons for each family and name them.
3. c) Explain why the molecular formula alone is not enough to determine which one it is, and what additional information (for example, whether it has a ring or an open chain) would help to identify it.

A student writes the names '3-methylbutane' and '4,4-dimethylpentane' in their exam.

1. a) Write the structural formula of the compound they intended to name in each case.
2. b) Explain the numbering error that was made.
3. c) Give the correct IUPAC names and state the rule of lowest locants.

A hydrocarbon has 6 carbons in the main chain, a double bond between carbons 2 and 3, and a methyl group on carbon 4.

1. a) Write its structural formula.
2. b) Name it according to IUPAC.
3. c) Check that it satisfies the general formula for alkenes and calculate its molar mass. Given atomic masses: $\text{C} = 12$ and $\text{H} = 1$.