Organic chemistry

Section 4.7

12 spec leafs

Notes and three levels of exam-style practice for each registered specification leaf in this section.

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4.7.1.1

Crude oil, hydrocarbons and alkanes

Crude oil is a finite resource found in rocks. It formed from ancient biomass, mainly plankton, that was buried in mud.
Crude oil is a mixture containing many compounds; most are hydrocarbons, whose molecules contain carbon and hydrogen atoms only.
Most crude-oil hydrocarbons are alkanes. Their general formula is $\mathrm{C_nH_{2n+2}}$ , and the first four are methane, ethane, propane and butane.
To test an alkane formula, substitute its carbon count for $n$ and check the hydrogen count. A common error is to call crude oil one compound rather than a mixture.

Tier 1 · Easy

2 marks

ORIGINAL

A molecule has the formula $\mathrm{C_3H_8}$ . State why it is a hydrocarbon and show that it fits the alkane general formula.

Tier 2 · Standard

3 marks

ORIGINAL

An alkane molecule contains seven carbon atoms. Determine its molecular formula and explain how you obtained the hydrogen count.

Tier 3 · Hard

4 marks

ORIGINAL

An alkane has relative molecular mass $58$ . Using $A_r(\mathrm{C})=12$ and $A_r(\mathrm{H})=1$ , determine its molecular formula and name it.

4.7.1.2

Fractional distillation and petrochemicals

Fractional distillation separates crude oil into fractions; each fraction contains hydrocarbons with similar numbers of carbon atoms and similar boiling points.
Crude oil is heated so that most of it vaporises. The column is hot at the bottom and cooler towards the top, so vapours condense at different heights.
Large molecules with higher boiling points condense lower in the column; smaller molecules with lower boiling points rise further before condensing.
Fractions provide fuels and petrochemical feedstock for solvents, lubricants, polymers and detergents. Fractional distillation separates molecules; it does not crack them.

Tier 1 · Easy

2 marks

ORIGINAL

What is meant by a fraction obtained from crude oil, and which physical property allows the fractions to be separated?

Tier 2 · Standard

3 marks

ORIGINAL

Hydrocarbon A has a boiling point of $75\,^\circ\mathrm{C}$ and hydrocarbon B has a boiling point of $210\,^\circ\mathrm{C}$ . Explain which one condenses lower in a fractionating column.

Tier 3 · Hard

5 marks

ORIGINAL

Describe how a heated sample of crude oil is separated in a fractionating column, then explain why the collected fractions are valuable even when they are not used directly as fuels.

4.7.1.3

Properties of hydrocarbons

As hydrocarbon molecules become larger, their boiling points increase because the intermolecular forces between molecules become stronger.
Increasing molecular size also increases viscosity and decreases flammability, influencing which fractions make convenient fuels.
Complete combustion releases energy and oxidises the carbon and hydrogen, producing carbon dioxide and water.
Balance combustion equations in the order carbon, hydrogen, then oxygen. Do not confuse incomplete combustion products with the specified products of complete combustion.

Tier 1 · Easy

2 marks

ORIGINAL

State how boiling point and viscosity change as the molecules in a homologous series of hydrocarbons become larger.

Tier 2 · Standard

2 marks

ORIGINAL

Complete and balance the equation for the complete combustion of propane: $\mathrm{C_3H_8 + O_2 \rightarrow CO_2 + H_2O}$ .

Tier 3 · Hard

5 marks

ORIGINAL

Fuel P is $\mathrm{C_5H_{12}}$ and fuel Q is $\mathrm{C_{15}H_{32}}$ . Compare their boiling points, viscosities and flammabilities, and write a balanced equation for the complete combustion of Q.

4.7.1.4

Cracking and alkenes

Cracking breaks long-chain hydrocarbons into smaller, more useful molecules because demand for short-chain fuels is high.
In catalytic cracking, a hydrocarbon is vaporised and passed over a hot catalyst; in steam cracking, hydrocarbon vapour is mixed with steam and heated strongly.
Cracking produces a mixture that includes shorter alkanes and alkenes. Equations must conserve the number of carbon and hydrogen atoms.
Alkenes decolourise bromine water from orange to colourless and are useful for polymers and other chemicals. Heating alone does not identify an alkene.

Tier 1 · Easy

1 mark

ORIGINAL

A colourless hydrocarbon rapidly turns orange bromine water colourless. What type of hydrocarbon is present?

Tier 2 · Standard

3 marks

ORIGINAL

Complete the cracking equation $\mathrm{C_{12}H_{26} \rightarrow C_8H_{18} + X}$ and identify the homologous series of X.

Tier 3 · Hard

5 marks

ORIGINAL

A refinery converts $\mathrm{C_{15}H_{32}}$ into $\mathrm{C_9H_{20}}$ and one other product. Give the other product, describe catalytic cracking, and explain two reasons the process is useful.

4.7.2.1

Structure and formulae of alkenes (chemistry only)

Alkenes are hydrocarbons containing a carbon-carbon double bond, C=C; this double bond makes them unsaturated.
Their homologous-series formula is $\mathrm{C_nH_{2n}}$ , so an alkene has two fewer hydrogen atoms than the alkane with the same carbon count.
The first four members are ethene, propene, butene and pentene, and each may be shown by a molecular or fully displayed structural formula.
When recognising an alkene, check both the double bond and the formula. A common error is to apply the alkane formula $\mathrm{C_nH_{2n+2}}$ .

Tier 1 · Easy

1 mark

ORIGINAL

Use the general formula of the alkenes to give the molecular formula of pentene.

Tier 2 · Standard

2 marks

ORIGINAL

A displayed molecule contains three carbon atoms, one C=C bond and six hydrogen atoms. Name the molecule and explain why it is unsaturated.

Tier 3 · Hard

5 marks

ORIGINAL

An alkene has relative molecular mass $56$ . Using $A_r(\mathrm{C})=12$ and $A_r(\mathrm{H})=1$ , determine its formula and name, then give the formula of the alkane with the same number of carbon atoms.

4.7.2.2

Reactions of alkenes (chemistry only)

Alkene reactions are governed by the C=C functional group: addition places atoms across the double bond and leaves a single carbon-carbon bond.
Hydrogen adds in the presence of a nickel catalyst to form an alkane; steam adds with a phosphoric acid catalyst to form an alcohol.
Chlorine, bromine and iodine add across C=C to form saturated dihalogeno compounds; bromine water changes from orange to colourless.
Alkenes combust like other hydrocarbons but often burn with smoky flames in air because combustion is incomplete. Do not leave C=C in an addition product.

Tier 1 · Easy

2 marks

ORIGINAL

Ethene is shaken with bromine water. State the observed colour change and the type of reaction.

Tier 2 · Standard

3 marks

ORIGINAL

Draw the fully displayed structural formula of the product when ethene reacts with hydrogen. Name the product and state the catalyst used.

Tier 3 · Hard

6 marks

ORIGINAL

A hydrocarbon burns with a smoky flame and decolourises bromine water. Explain both observations, then describe how it can be converted into an alcohol and name the product when the hydrocarbon is ethene.

4.7.2.3

Alcohols (chemistry only)

Alcohols contain the -OH functional group; the first four are methanol, ethanol, propanol and butanol.
They burn in air, react with sodium and are oxidised to carboxylic acids. Their solubility in water decreases as the carbon chain becomes longer.
Ethanol is made by fermenting a sugar solution with yeast under warm conditions without oxygen; fermentation also produces carbon dioxide.
Alcohols are used as fuels and solvents. Balanced equations are required for their combustion, but not for their other reactions in this specification.

Tier 1 · Easy

2 marks

ORIGINAL

Name $\mathrm{CH_3CH_2OH}$ and identify its functional group.

Tier 2 · Standard

4 marks

ORIGINAL

A student wants to produce ethanol from glucose solution by fermentation. State the biological agent, two required conditions and the other product formed.

Tier 3 · Hard

5 marks

ORIGINAL

Ethanol is first burned completely and a separate sample is oxidised. Write the balanced combustion equation, name the oxidation product and describe the role of the oxidising agent.

4.7.2.4

Carboxylic acids (chemistry only)

Carboxylic acids contain the -COOH functional group; the first four are methanoic, ethanoic, propanoic and butanoic acid.
They dissolve in water to form acidic solutions and react with carbonates to make a salt, water and carbon dioxide. Higher only: carboxylic acids are weak because they only partially ionise, so an equally concentrated strong acid has a lower pH.
A carboxylic acid reacts with an alcohol to form an ester and water; ethanol with ethanoic acid forms ethyl ethanoate.
Recognise the -COOH group before naming the molecule. A common error is to identify it as the -OH group of an alcohol.

Tier 1 · Easy

2 marks

ORIGINAL

Name $\mathrm{CH_3COOH}$ and state the functional group that identifies it as a carboxylic acid.

Tier 2 · Standard

4 marks

ORIGINAL

Dilute ethanoic acid is added to calcium carbonate. Describe the observation and name all three types of product.

Tier 3 · Hard

5 marks

ORIGINAL

Two colourless liquids are ethanol and ethanoic acid. Describe a carbonate test that distinguishes them, then name the organic product when the two liquids react together.

4.7.3.1

Addition polymerisation (chemistry only)

In addition polymerisation, many alkene monomers join to form one long-chain polymer molecule.
The monomer's C=C bond opens to form C-C bonds linking neighbouring repeating units; no small molecule is produced.
The repeating unit contains the same atoms as the monomer and is shown in brackets with continuation bonds and a subscript $n$ .
To recover the monomer from a repeating unit, place C=C between the two backbone carbons. Do not leave a double bond in the addition polymer.

Tier 1 · Easy

2 marks

ORIGINAL

Name the monomer used to make poly(ethene) and state the type of polymerisation.

Tier 2 · Standard

3 marks

ORIGINAL

Propene, $\mathrm{CH_2{=}CHCH_3}$ , forms a polymer. Write its repeating unit and explain what happens to the carbon-carbon double bond.

Tier 3 · Hard

4 marks

ORIGINAL

A polymer has repeating unit $[\mathrm{-CH_2-CHCl-}]_n$ . Deduce the monomer formula and explain why no other product forms during polymerisation.

4.7.3.2

Condensation polymerisation (chemistry only) (HT only)

Condensation polymerisation requires monomers with two functional groups, allowing each monomer to join at both ends of a growing chain.
When a link forms, a small molecule such as water is usually eliminated; the polymer therefore does not contain every atom from the monomers.
A diol and a dicarboxylic acid form a polyester because -OH groups react with -COOH groups to make ester links.
Read the repeating unit across the new links and preserve the remaining carbon chains. A common error is to treat condensation as C=C addition.

Tier 1 · Easy

3 marks

ORIGINAL

State two features required for condensation polymerisation and name a small molecule commonly released.

Tier 2 · Standard

4 marks

ORIGINAL

Ethanediol reacts with butanedioic acid. Identify the two functional-group types, name the polymer class and state the small molecule eliminated.

Tier 3 · Hard

5 marks

ORIGINAL

Use the monomers $\mathrm{HO-CH_2-CH_2-OH}$ and $\mathrm{HOOC-CH_2-CH_2-COOH}$ to write one repeating unit of the polyester and explain how its links form.

4.7.3.3

Amino acids (chemistry only) (HT only)

An amino acid contains two different functional groups in one molecule: an amino group, -NH2, and a carboxyl group, -COOH.
Glycine is H2NCH2COOH. Amino and carboxyl groups on different molecules undergo condensation to form peptide links.
Each peptide link, -CONH-, forms with the loss of water, and repeated condensation produces a polypeptide.
Different amino acids can occur in the same chain, producing proteins. Do not describe this as addition polymerisation because no C=C bond opens.

Tier 1 · Easy

3 marks

ORIGINAL

Identify the two functional groups in $\mathrm{H_2NCH_2COOH}$ and name this amino acid.

Tier 2 · Standard

4 marks

ORIGINAL

Two glycine molecules condense. Write the structural formula of the product, identify the new link and state the other product.

Tier 3 · Hard

5 marks

ORIGINAL

Four amino acid molecules join to make one unbranched polypeptide molecule. Determine the number of peptide links and water molecules formed, then explain how using different amino acids can change the polymer.

4.7.3.4

DNA (deoxyribonucleic acid) and other naturally occurring polymers (chemistry only)

DNA is a large molecule essential for life because it encodes genetic instructions for the development and functioning of organisms and viruses.
Most DNA consists of two polymer chains arranged as a double helix and built from four different monomers called nucleotides.
Proteins are polymers of amino acids, while starch and cellulose are polymers made from glucose monomers.
Match each natural polymer to its own monomer type. A common error is to describe DNA as a protein or to say that amino acids are its monomers.

Tier 1 · Easy

2 marks

ORIGINAL

Name the monomer type from which DNA is made and state how many different monomers are used.

Tier 2 · Standard

4 marks

ORIGINAL

Match each natural polymer to its monomer type: protein, starch, cellulose and DNA.

Tier 3 · Hard

5 marks

ORIGINAL

A student claims that DNA, proteins and starch must share one monomer because all three occur naturally. Evaluate the claim using monomers and one structural feature of DNA.