Hydrocarbons: Chemistry 9701 (Cambridge International AS & A Level)

Butane, $\text{C}_4\text{H}_{10}$, is the fuel used in a portable camping stove. When the stove burns with a steady blue flame, there is a plentiful supply of air and combustion is complete.

Which statement about the combustion of butane is correct?

Ethane, $\text{CH}_3\text{CH}_3$, reacts slowly with bromine vapour when the mixture is exposed to ultraviolet light, forming bromoethane as the main organic product alongside hydrogen bromide gas. A small amount of butane, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_3$, is also detected among the products.

(a) State the type of mechanism occurring, and the essential condition needed to initiate it. [2]

(b) Write an equation for the initiation step of this mechanism, and state the type of bond fission that occurs. [2]

(c) Write equations for the two propagation steps that together convert ethane and bromine into bromoethane and hydrogen bromide. [2]

(d) Write an equation for one termination step that would account for the butane detected among the products, and explain why several different termination products (not just bromoethane) are obtained overall. [2]

But-1-ene, $\text{CH}_3\text{CH}_2\text{CH}=\text{CH}_2$, reacts with hydrogen bromide gas at room temperature to give a mixture of two isomeric bromobutanes, with one isomer strongly predominating.

(a) State the type of mechanism for this reaction. [1]

(b) Describe, in words, how this mechanism proceeds, including the structure of any intermediate(s) formed, and use it to explain which of the two possible bromobutane products is obtained as the major product. [4]

(c) But-1-ene also reacts with bromine water. Describe what would be observed, and explain how this observation confirms that but-1-ene contains a carbon-carbon double bond. [2]

Benzene reacts with chlorine gas in the presence of anhydrous aluminium chloride, $\text{AlCl}_3$, a halogen carrier catalyst, forming chlorobenzene by electrophilic substitution.

Which row correctly identifies the electrophile generated in this reaction, and the immediate fate of the arenium ion (the non-aromatic, positively charged intermediate formed once the electrophile has attacked the ring)?

Ethylbenzene, $\text{C}_6\text{H}_5\text{CH}_2\text{CH}_3$, is nitrated using a mixture of concentrated nitric acid and concentrated sulfuric acid at about 50°C. The product is predominantly a mixture of 2-nitroethylbenzene and 4-nitroethylbenzene, with only a very small amount of 3-nitroethylbenzene formed.

(a) Write an equation, using the two concentrated acids, to show how the electrophile involved in this nitration is generated, and identify the electrophile. [2]

(b) Describe, in words, the mechanism by which this electrophile reacts with the aromatic ring of ethylbenzene to form a nitro product, including the intermediate formed and how the ring's aromaticity is restored afterwards. [3]

(c) Explain why the ethyl group in ethylbenzene directs the incoming electrophile mainly to the 2- and 4-positions rather than the 3-position, and why ethylbenzene reacts faster than benzene itself under the same conditions. [3]

Propane, $\text{CH}_3\text{CH}_2\text{CH}_3$, is reacted with chlorine gas, $\text{Cl}_2$, in the presence of ultraviolet light. Monochlorination (substitution of just one hydrogen atom per molecule) can occur at either a primary carbon or the secondary carbon, giving a mixture of two structural isomers of chloropropane.

Which statement about this reaction is correct?

But-2-ene, $\text{CH}_3\text{CH}=\text{CHCH}_3$, is an unsaturated hydrocarbon that can be converted into butane by hydrogenation.

(a) Identify the type of reaction that converts but-2-ene into butane, and state the reagent and conditions required. [3]

(b) Write a balanced equation for this reaction. [1]

(c) Give one industrial application of the hydrogenation of carbon-carbon double bonds in unsaturated compounds. [1]

(d) Explain, in terms of bonding, why but-2-ene can undergo this reaction but butane cannot react further with hydrogen. [2]

2-Methylpropene, $\text{CH}_2=\text{C}(\text{CH}_3)_2$, reacts with steam in the presence of a catalyst of concentrated phosphoric(V) acid, $\text{H}_3\text{PO}_4$, to give a mixture of two isomeric alcohols, with one isomer strongly predominating.

(a) State the type of mechanism for this reaction. [1]

(b) Describe, in words, how this mechanism proceeds, including the structure of any intermediate(s) formed, and use it to explain which of the two possible alcohol products is obtained as the major product. [4]

(c) Write an equation for the reaction between the major carbocation intermediate and water, give the further step needed to form the neutral major alcohol product, and explain why the $\text{H}_3\text{PO}_4$ catalyst is not used up overall. [3]

Propene, $\text{CH}_3\text{CH}=\text{CH}_2$, is shaken with orange bromine water, $\text{Br}_2(\text{aq})$, at room temperature in the absence of ultraviolet light.

Which statement about this reaction is correct?

Decane, $\text{C}_{10}\text{H}_{22}$, obtained from the fractional distillation of crude oil, is in lower demand than the shorter, more volatile hydrocarbons used as fuels. It is converted into more useful products by cracking.

(a) State the two general types of cracking, and give typical conditions for each. [2]

(b) Write a balanced equation for the thermal cracking of decane into octane, $\text{C}_8\text{H}_{18}$, and one other product. [2]

(c) State the type of bond fission that occurs when a C-C bond breaks during thermal cracking, and explain why cracking is industrially important in terms of the supply of hydrocarbon fractions from crude oil. [2]