Introduction to Organic Chemistry: Chemistry 9701 (Cambridge International AS & A Level)

An organic chemist draws the structural formula of a branched, brominated alkane:

$\text{CH}_3\text{CH(CH}_3\text{)CH}_2\text{CHBrCH}_3$

What is the correct IUPAC name for this compound?

Compounds P, Q, R and S below all have the same molecular formula, $\text{C}_4\text{H}_8$.

• P: $\text{CH}_2=\text{CHCH}_2\text{CH}_3$
• Q: $\text{CH}_3\text{CH}=\text{CHCH}_3$
• R: $(\text{CH}_3)_2\text{C}=\text{CH}_2$
• S: cyclobutane, a four-membered ring of carbon atoms joined only by single C-C bonds (no C=C double bond)

(a) State the general formula, in terms of $n$, of the homologous series of non-cyclic alkenes containing one C=C double bond, and use it to confirm that $\text{C}_4\text{H}_8$ is consistent with compounds P, Q and R. [2]

(b) Give the IUPAC name of compound R, and state the type of functional group present in P, Q and R that is completely absent from compound S. [2]

(c) State the type of structural isomerism (chain, positional or functional group) shown by each of the following pairs of compounds, giving a brief reason for each answer:

(i) P and Q [1]

(ii) P and R [1]

(iii) P and S [1]

(d) State the structural condition that must be satisfied at each carbon atom of a C=C double bond for cis-trans (geometrical) isomerism to be possible, and use it to determine, with a reason, which one of P, Q and R can show this type of isomerism. [2]

A covalent bond between two atoms, A and B, can break in two different ways, known as homolytic fission and heterolytic fission.

Which statement correctly describes these two types of bond fission and the particles produced?

A chemist is investigating compound Q, 2-methylpentan-3-ol, $\text{CH}_3\text{CH(CH}_3\text{)CH(OH)CH}_2\text{CH}_3$, as a possible chiral starting material for drug synthesis.

(a) State the structural criterion for a carbon atom to be described as a chiral centre. Identify, giving a reason based on the four groups attached to it, whether C3 of compound Q is a chiral centre. [2]

(b) The two enantiomers of compound Q are non-superimposable mirror images of each other. Describe a physical technique that could distinguish between separate, pure samples of the two enantiomers, and state what would be observed if this technique were instead applied to a racemic (1:1) mixture of the two enantiomers. [2]

(c) Explain why the two enantiomers of compound Q have identical melting points and boiling points, yet can behave very differently when interacting with other chiral molecules such as those found in the human body, and explain why this matters when a chiral compound is developed as a drug. [3]

A benzene ring bears one chlorine atom and one methyl group. The two substituents are positioned meta to each other, that is, separated by one unsubstituted ring carbon on the shorter path around the ring.

What is the correct IUPAC name of this compound?

A student draws the skeletal formula of a hydrocarbon. It shows a five-carbon zigzag chain in which the first carbon-carbon bond, at the left-hand end of the chain, is drawn as a double line (a C=C double bond); a two-carbon ethyl branch is attached to the third carbon of the chain, counting from the left. There are no other branches, rings or functional groups.

What is the molecular formula of this compound?

A technician carries out a sequence of four organic reactions.

(a) Ethanol, $\text{CH}_3\text{CH}_2\text{OH}$, is heated with an excess of concentrated sulfuric acid, causing it to lose a molecule of water and form ethene, $\text{CH}_2=\text{CH}_2$. State the type of reaction (addition, substitution, elimination, oxidation, reduction, hydrolysis or condensation) and give a reason for your classification based on what happens to the molecule. [2]

(b) The ethene formed in (a) is then reacted with bromine, $\text{Br}_2$, forming 1,2-dibromoethane, $\text{CH}_2\text{BrCH}_2\text{Br}$. State the type of reaction and give a reason for your classification. [2]

(c) The 1,2-dibromoethane from (b) is warmed with aqueous sodium hydroxide, and each bromine atom is replaced by a hydroxyl group, forming ethane-1,2-diol, $\text{CH}_2(\text{OH})\text{CH}_2(\text{OH})$. State the type of reaction and give a reason for your classification. [2]

(d) The ethane-1,2-diol from (c) is then reacted with ethanoic acid, $\text{CH}_3\text{COOH}$; the two molecules join together, losing one small molecule overall, to form an ester linkage. State the type of reaction and name the small molecule that is lost. [1]

Reaction mechanisms in organic chemistry are often described as electrophilic, nucleophilic or free-radical, depending on the species that attacks the substrate.

Which statement correctly defines the terms nucleophile and electrophile?

Compound Z is 2-bromobut-2-ene, $\text{CH}_3\text{C(Br)}=\text{CHCH}_3$, numbering the four carbons C1 to C4 along the chain so that the double bond lies between C2 and C3. This compound exists as a pair of E/Z stereoisomers.

(a) State the Cahn-Ingold-Prelog (CIP) priority rule used to rank the two substituents attached to each carbon of a stereogenic C=C double bond, and use it to identify the higher-priority group on C2 and the higher-priority group on C3 of compound Z. [3]

(b) In one particular sample of compound Z, the bromine atom on C2 and the methyl group attached to C3 lie on opposite sides of the double bond. Using your answer to part (a), determine whether this sample is the E isomer or the Z isomer, explaining your reasoning. [2]

(c) But-2-ene, $\text{CH}_3\text{CH}=\text{CHCH}_3$, can also be described using either cis/trans notation or E/Z notation. State, with a reason, whether the two notations give equivalent information for but-2-ene specifically. [2]

A chloroalkane has molecular formula $\text{C}_4\text{H}_9\text{Cl}$. Four constitutional (structural) isomers share this molecular formula:

• J: $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{Cl}$ (1-chlorobutane)
• K: $\text{CH}_3\text{CH}_2\text{CH(Cl)CH}_3$ (2-chlorobutane)
• L: $(\text{CH}_3)_2\text{CHCH}_2\text{Cl}$ (1-chloro-2-methylpropane)
• M: $(\text{CH}_3)_3\text{CCl}$ (2-chloro-2-methylpropane)

(a) State the general formula, in terms of $n$, of the homologous series of monochloroalkanes (compounds with one chlorine atom, no ring and no C=C bond), and use it to confirm that $\text{C}_4\text{H}_9\text{Cl}$ is consistent with this formula for $n=4$. [2]

(b) State the type of structural isomerism relating J and K, and the type relating J and M. [2]

(c) Identify, giving a reason based on the four groups attached, which one (if any) of J, K, L and M contains a chiral centre. [2]