12th Class Chemistry Hydroxy Compounds Short Questions

Q2.a.i.Six alcohols are given below: i. butan-2-ol ii. ethane-1,2-diol iii. 2-methylpentan-3-ol iv. 2-methylpropan-2-ol v. propan-1-ol vi. propan-2-ol. Which alcohol is an example of a tertiary alcohol?

iv. 2-methylpropan-2-ol, (CH₃)₃COH. Its –OH is attached to a carbon that is bonded to three other carbons (three methyl groups), the general formula R₃COH, which defines a tertiary alcohol.

Q2.a.ii.Which alcohol (from the list in Q2.a) is an example of an alcohol that can be oxidized to carboxylic acid?

ii. ethane-1,2-diol and v. propan-1-ol are both primary alcohols (R-CH₂OH); primary alcohols are oxidised first to an aldehyde and, on further (excess) oxidation with acidified K₂Cr₂O₇ or KMnO₄, to the corresponding carboxylic acid.

Q2.a.iii.Which alcohol (from the list in Q2.a) is an example of an alcohol that can be oxidized to a ketone?

i. butan-2-ol, iii. 2-methylpentan-3-ol and vi. propan-2-ol are all secondary alcohols (R₂CHOH); secondary alcohols are oxidised by acidified K₂Cr₂O₇/KMnO₄ to ketones.

Q2.a.v.Which alcohol (from the list in Q2.a) can't change colour of acidified potassium dichromate?

Q2.b.i.Phenylamine reacts with HNO₂(aq) at 4°C to form compound P. Compound P reacts with phenol under alkaline conditions at 4°C. The product of this reaction is acidified, forming azo compound Q. Draw the structure of compound P and Q.

Q2.b.ii.Circle the azo group on your structure (of compound Q).

The azo group is the –N=N– linkage joining the two aromatic rings in Q, i.e. the nitrogen–nitrogen double bond between the phenyl ring (from P) and the hydroxyphenyl ring (from phenol): C₆H₅–[N=N]–C₆H₄–OH.

Q2.b.iii.State one use of an azo compound such as Q.

Azo compounds such as Q are used as synthetic dyes/colourants, for example to dye textiles and to colour foods.

Q2.c.i.Cyclohexane can be converted into cyclohexene via a three stage synthesis. [Scheme: cyclohexane --stage 1, Cl₂--> compound A --stage 2--> cyclohexanol --stage 3--> cyclohexene] In stage 1, cyclohexane reacts with chlorine to form the organic product, compound A. Show the structure of compound A.

Compound A is chlorocyclohexane, C₆H₁₁Cl — a cyclohexane ring with one hydrogen replaced by a chlorine atom, formed by free-radical substitution of cyclohexane with Cl₂ (UV light).

Q2.c.ii.Stage 3 involves the dehydration of an alcohol. State a suitable reagent for dehydrating an alcohol.

Heating cyclohexanol with concentrated sulfuric acid (H₂SO₄) or concentrated phosphoric acid (H₃PO₄); alternatively, passing the alcohol vapour over a heated aluminium oxide (Al₂O₃) catalyst.

Q2.c.iii.Write a balanced equation for the dehydration of cyclohexanol, C₆H₁₁OH.

C₆H₁₁OH → C₆H₁₀ + H₂O (heated with conc. H₂SO₄ or Al₂O₃ catalyst)

Q2.d.i.There are four structurally isomeric alcohols of molecular formula C₄H₁₀O. [Graphical formulae of isomers A, B, C and D are shown: A is (CH₃)₃C–OH; B is CH₃CH₂CH(OH)CH₃; C is CH₃CH₂CH₂CH₂OH; D is (CH₃)₂CHCH₂OH.] Identify the type of alcohol represented by A and by B.

A, (CH₃)₃C–OH, is a tertiary alcohol (the –OH carbon is bonded to three other carbons). B, CH₃CH₂CH(OH)CH₃, is a secondary alcohol (the –OH carbon is bonded to two other carbons).

Q2.d.ii.Give the name of alcohol A.

2-methylpropan-2-ol (tert-butyl alcohol).

Q2.d.iii.Select one of the alcohols A, B, C or D which will, on oxidation, produce an aldehyde. Give the structural formula of the aldehyde produced by this reaction.

C, butan-1-ol, CH₃CH₂CH₂CH₂OH, is a primary alcohol, so oxidation (e.g. by distillation with acidified K₂Cr₂O₇) gives the aldehyde butanal: CH₃CH₂CH₂CHO. (D, 2-methylpropan-1-ol, is also primary and would equally oxidise to an aldehyde, 2-methylpropanal.)

Q2.d.iv.All the alcohols A, B, C and D may be readily dehydrated. Explain what is meant by the term dehydration. State the type of compound formed by dehydration of alcohols.

Dehydration is the removal of a water molecule from an alcohol; it is an elimination reaction. Dehydrating an alcohol produces an alkene.

Q2.d.v.Suggest suitable reagent(s) and condition(s) for the dehydration of alcohols.

Heat the alcohol with concentrated sulfuric acid (H₂SO₄) or concentrated phosphoric acid (H₃PO₄); alternatively, pass the alcohol vapour over a heated aluminium oxide (Al₂O₃) catalyst.

Q2.e.Select one of the alcohols A, B, C or D which, on dehydration, would give a single product. Draw the structural formula of this product.

Q3.I.Why nitration of phenol is easier than benzene? Which product is formed when phenol is nitrated with conc. HNO₃ at room temperature?

The –OH group of phenol donates one of oxygen's lone pairs into the aromatic ring (delocalisation into the π system), increasing the electron density of the ring. This activates the ring towards electrophilic attack, so phenol undergoes nitration far more readily (under milder conditions) than benzene. When phenol is nitrated with concentrated HNO₃ at room temperature, the product is 2,4,6-trinitrophenol.

Q3.ii.Sodium hydroxide reacts with phenol but not with ethanol. Give reason.

Phenol is markedly more acidic than ethanol because its conjugate base, the phenoxide ion, is stabilised by delocalisation of the negative charge on oxygen into the aromatic ring, whereas ethanol's conjugate base, the ethoxide ion, is destabilised by the electron-donating (+I) ethyl group. Phenol is therefore acidic enough to be deprotonated by the weak base NaOH, forming soluble sodium phenoxide and water, while ethanol (less acidic than even water) cannot be deprotonated by NaOH.

Q3.iii.Give reactions of methanol with SOCl₂, and PCl₅.

CH₃OH + SOCl₂ → CH₃Cl + SO₂ + HCl
CH₃OH + PCl₅ → CH₃Cl + POCl₃ + HCl

Q3.iv.How can propanol be converted into propanal?

Propan-1-ol (a primary alcohol) is oxidised to propanal by heating with acidified K₂Cr₂O₇ (or KMnO₄) and distilling off the aldehyde as it forms (before it is further oxidised to propanoic acid): CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O.

Q3.v.Write down the reaction for the substitution of each of Cl, Br, and I with –OH in 2-propanol.

Q3.vi.Construct a reaction for the preparation of nitro-substituted naphthol. Is there produced a single product? Explain.

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Unit 22: Hydroxy Compounds — Short Questions