Organic Chemistry — practice questions

Name the functional group present in each of: (a) CH₃CH₂CHO, (b) CH₃COCH₃, (c) CH₃CH₂COCl, (d) CH₃CH₂CONH₂.

Explain how you can tell from its structure whether a compound containing a C=O group is an aldehyde or a ketone, and explain why this distinction affects how the compound reacts with acidified dichromate.

Consider the three compounds CH₃CH₂COOH, CH₃CH₂COOCH₃ and CH₃CH₂CONH₂. All three contain a carbonyl carbon bonded to a second heteroatom. Compare and contrast their structures, and justify why only one of them will produce bubbles of carbon dioxide when sodium carbonate solution is added.

Give the IUPAC name of (a) CH₃CH₂CH₂CHO and (b) CH₃COCH₂CH₃, and draw the structural formula of propanoyl chloride.

Name the compound CH₃CH(OH)CH₂COOH, and explain how you decided which functional group provides the suffix and which becomes a prefix.

A student names the compound CH₃CH₂COOCH₂CH₂CH₃ as 'propyl propanoate' and the compound CH₃CH₂CH₂COOCH₂CH₃ as 'propyl ethanoate'. One name is correct and one is not. Identify the error, give the correct name, and explain the general rule that the student has misapplied. Support your answer by drawing both compounds.

Define the term 'constitutional isomer' and draw two constitutional isomers of C₃H₈O.

Propanal and propanone are constitutional isomers with the molecular formula C₃H₆O. Explain how a simple chemical test could distinguish between them, and explain why the test works.

Draw all four constitutional isomers of C₄H₉Br, name each, and classify each as primary, secondary or tertiary. Predict which isomers would give the same organic product on treatment with KOH in alcohol, and justify your prediction.

State the two conditions a molecule must meet to show cis–trans isomerism, and state whether but-1-ene, CH₂=CHCH₂CH₃, meets them.

Explain why but-2-ene exists as cis and trans isomers but butane does not, even though both have four carbons in a chain.

The compound CH₃CH=CHCOOH exists as cis and trans isomers. The trans isomer melts at 72 °C and the cis isomer at 14 °C, and the cis isomer is the more soluble in water. Justify both observations in terms of the structures of the two isomers, and explain why a chemical test could not be used to distinguish them.

Define a chiral carbon, and identify the chiral carbon in CH₃CH₂CH(NH₂)COOH.

Explain why 2-bromobutane exists as enantiomers but 1-bromobutane does not.

A chemist synthesises 2-chlorobutane from but-1-ene and obtains a product that does not rotate plane-polarised light at all. A colleague concludes that the product cannot contain a chiral carbon and must therefore be 1-chlorobutane. Evaluate this conclusion, and explain how the two possibilities could be distinguished experimentally.

State whether each of butan-1-ol, butane and butanoic acid is soluble in water, and identify which has the highest boiling point.

Propanone, CH₃COCH₃, is very soluble in water but has a boiling point of only 56 °C, much lower than propan-1-ol at 97 °C. Explain both observations.

A mixture contains hexanoic acid and hexan-1-ol, which have boiling points of 205 °C and 157 °C. A student proposes separating them by distillation. Evaluate this proposal, and describe an alternative method based on solubility, justifying why it would work.

State the reagent and conditions needed to convert (a) 1-bromobutane into butan-1-ol, and (b) butan-1-ol into but-1-ene.

Describe a two-step route to convert propan-2-ol into propanone, then into propan-2-ol again. Give the reagents and conditions for each step and explain why the second step is called a reduction.

Devise a reaction scheme to convert 1-bromopropane into propyl propanoate. For each step give the reagents, the conditions and the structural formula of the organic product. Justify why your scheme requires the starting material to be used in two separate portions.

Write equations for the reaction of 1-chloropropane with (a) NaOH(aq) and (b) KOH in alcohol, and name the organic product in each case.

Explain why 1-bromobutane gives a different product with KOH(aq) than with KOH dissolved in ethanol, and identify both products.

A student reacts 2-bromo-2-methylpropane, (CH₃)₃CBr, with concentrated ammonia in a sealed tube and obtains a poor yield of the expected amine, along with a substantial amount of an alkene. Explain the formation of the alkene, and evaluate whether increasing the concentration of ammonia would improve the yield of the amine. Compare this outcome with what you would expect from 1-bromobutane.

Write equations for the reaction of propan-1-ol with (a) HBr and (b) concentrated H₂SO₄ with heating, and name the organic product in each case.

Explain why thionyl chloride, SOCl₂, is often preferred to concentrated HCl for converting an alcohol into a chloroalkane.

Concentrated sulfuric acid is used both to dehydrate an alcohol to an alkene and as the catalyst in esterification. Explain how the same reagent can play these two different roles, and justify how a chemist would set up each reaction to obtain the intended product rather than the other one.

2-bromobutane is heated with KOH dissolved in ethanol. Name the major and the minor organic products.

Pentan-2-ol is heated with concentrated sulfuric acid. Draw the two organic products, identify which is the major product, and explain your reasoning.

A student eliminates HBr from an unknown bromoalkane of formula C₅H₁₁Br using KOH in ethanol, and obtains only a single alkene. A second student uses a different isomer of C₅H₁₁Br and obtains two alkenes. Identify a bromoalkane consistent with each result, justify your choices, and explain what this shows about when the major/minor rule applies.

Describe the test for a carbon–carbon double bond, stating the reagent and the observation, and write the equation for the reaction of but-2-ene with this reagent.

An alkene of formula C₄H₈ reacts with bromine water to give 1,2-dibromobutane. Identify the alkene and explain how the structure of the product allowed you to deduce it.

A student heats an unknown bromoalkane of formula C₄H₉Br with KOH in ethanol and obtains a mixture of two alkenes. Treating the mixture with bromine water gives two dibromides: 1,2-dibromobutane and 2,3-dibromobutane. Identify the original bromoalkane, justify your identification, and state which alkene was the major product with your reasoning.

State the product formed when each of propan-1-ol, propan-2-ol and 2-methylpropan-2-ol is heated under reflux with excess acidified dichromate, and give the observation in each case.

Explain why a primary alcohol can be oxidised to either an aldehyde or a carboxylic acid, and describe how the conditions are chosen to obtain the aldehyde.

A student oxidises butan-1-ol with acidified dichromate and collects a distillate. Testing the distillate, they find it gives a silver mirror with Tollens' reagent but also fizzes with sodium carbonate solution. Explain what these two observations together reveal about the distillate, evaluate the student's technique, and describe how the procedure could be improved.

Describe a test that distinguishes propanal from propanone, giving the reagent and the observation for each compound.

Explain why Tollens' reagent reacts with an aldehyde but not with a ketone or with a primary alcohol.

A compound Z of formula C₅H₁₀O gives no reaction with Tollens' reagent, but is reduced by NaBH₄ to a product that exists as enantiomers. Deduce a possible structure for Z, justify it fully, and explain why the Tollens' result alone would not have been sufficient to reach your answer.

Write equations for the reaction of propanoyl chloride with (a) water, (b) ethanol and (c) ammonia, and name the organic product in each case.

Describe a chemical test that distinguishes butanoic acid from butanoyl chloride, and explain why the test works.

A student is given a mixture of hexanoic acid, hexan-1-ol and hexyl hexanoate, all with six-carbon chains and all insoluble in water. Devise a scheme to separate all three, justifying each step, and explain why their similar chain lengths make a physical separation unattractive.

Write an equation for the reaction of ethanoyl chloride, CH₃COCl, with methylamine, CH₃NH₂, and state the type of reaction. Also write the equation for the reaction of ethylamine with hydrochloric acid.

Explain why propan-1-amine turns damp red litmus paper blue but propanamide does not, even though both contain an –NH₂ group.

A student wishes to prepare N-ethylpropanamide, CH₃CH₂CONHCH₂CH₃, starting from propan-1-ol and bromoethane as the only organic starting materials. Devise a full scheme with reagents, conditions and structures, and justify why the amide cannot be made by simply heating propanoic acid with ethylamine.

Give a chemical test, with observations, to distinguish ethanoic acid from ethanol.

Describe how you could distinguish between propan-1-ol, propanal and propanone using chemical tests. Give the reagents and all observations.

A single unknown liquid gives the following results: it decolourises bromine water, it fizzes with sodium carbonate solution, it gives no silver mirror with Tollens' reagent, and it has the molecular formula C₄H₆O₂. Deduce a structure for the unknown, justify each deduction, and explain why the negative Tollens' result was necessary rather than merely confirmatory.

Write equations for the hydrolysis of methyl propanoate, CH₃CH₂COOCH₃, with (a) dilute sulfuric acid and (b) aqueous sodium hydroxide, and name the organic products.

Explain why the base hydrolysis of an ester goes to completion but the acid hydrolysis does not.

A student refluxes an unknown compound of formula C₅H₁₁NO with aqueous sodium hydroxide and observes a gas that turns damp red litmus blue. On acidifying the remaining solution, a compound is obtained that fizzes with sodium carbonate and has the formula C₄H₈O₂. Deduce the structure of the unknown, justify each step, and explain what a repeat of the experiment using dilute hydrochloric acid instead would have produced.

Name the type of link formed in a polyester and in a polyamide, state the small molecule lost when a dicarboxylic acid reacts with a diol, and name this type of polymerisation.

The amino acid alanine has the structure H₂N–CH(CH₃)–COOH. Draw a section of the protein chain formed from three alanine molecules, and explain why alanine can polymerise with itself while ethylamine cannot.

Kevlar is a polyamide made from benzene-1,4-dicarboxylic acid and benzene-1,4-diamine. Poly(ethene) is made from ethene. Compare and contrast the two polymerisations, and justify why Kevlar can be broken down by hot concentrated acid but poly(ethene) cannot.

Name the alcohol from which all triglycerides are formed, state how many ester links a triglyceride contains, and name the products of its hydrolysis with hot aqueous sodium hydroxide.

Explain why a triglyceride made from unsaturated fatty acids is usually a liquid at room temperature, while one made from saturated fatty acids of similar chain length is a solid.

Explain how soap removes grease from a fabric, relating your answer to the structure of the soap molecule produced by saponification. Justify why the acid hydrolysis of a triglyceride would not produce an effective soap.

Give a two-step route to convert 2-bromopropane into propanone, stating the reagents and conditions and the intermediate product.

Devise a route to convert propan-1-ol into propyl propanoate, using propan-1-ol as the only organic starting material. Explain why the starting material must be divided into two portions.

Three students propose routes from propan-1-ol to ethyl propanoate, with ethanol also available.

Student A: oxidise propan-1-ol to propanoic acid, then react with ethanol and conc. H₂SO₄. Student B: oxidise propan-1-ol to propanal, then react with ethanol and conc. H₂SO₄. Student C: oxidise propan-1-ol to propanoic acid, convert to propanoyl chloride with SOCl₂, then react with ethanol.

Evaluate all three routes and justify which is best.