Chemistry_2017_STPM term 3_Objectives

The structural formula of an organic compound is shown. What are the types of hybrid orbitals formed by each atom labelled as x, y, z?

• x : sp²

  y : sp²

  z : sp

• x : sp²

  y : sp³

  z : sp

• x : sp³

  y : sp³

  z : sp²

• x : sp²

  y : sp³

  z : sp³

Answer explanation

The C atoms in a benzene ring undergo sp² hybridisation.

The single bond oxygen undergoes sp³ hybridisation.

Triple bonds involve sp hybridisation.

The empirical formula of an unsaturated hydrocarbon is CH₂. It exists as a liquid at room temperature. The chemical formula of the unsaturated hydrocarbon could be

• C₂H₄

• C₆H₁₂

• 
• C₁₈H₃₆

Answer explanation

2-Chlorobutatonoic acid is more acidic than 4-chlorobutatonoic acid because

• it has a higher solubility in water.

• its carboxylate anion is less stable.

• it forms a stronger conjugate base.

• of the more effective inductive effect of Cl atom towards the carboxyl group.

Answer explanation

Dissociation of carboxylic

acid:

RCOOH ↔ RCOO^- + H⁺

Chlorin is more electronegative than carbon and exerts an -I inductive effect that weakens the O — H bond in —COOH group. However, the inductive effect decreases the further away the chlorine atom is from the —COOH group. The Cl in 2-chlorobutanoic acid is closer to the carboxyl group thus exerting a greater inductive effect.

Toluene reacts with chlorine to form 4-chlorotoluene. Which statement is true about the reaction?

• The species formed in the initial step of the reaction is Cl⁺ ion.

• The reaction mechanisms is free radical substitution.

• The reaction also produces 3-chlorotoluene.

• The reaction is catalysed by UV light.

Answer explanation

The initial step involves the generation of the Cl⁺ ion.

Cl₂ + AlCl₃ → Cl⁺ + [ AlCl₄]⁻

This is an example of electrophilic substitution. The Cl^- ion acts as an electrophile which attacks the electron-rich benzene ring.

The reaction also produces 2-chlorotoluene. The —CH₃ is a 2, 4-director (ring activating group). Catalysis by ultraviolet light involves free-radical substitution to the —CH₃ side group to produce (chloromethyl) benzene.

Which alkene will form two different carboxylic acids when heated with acidified KMnO₄ solution?

• CH₃CH₂CH=CH₂

• (CH₃)₂C=CHCH₃

• CH₃CH=CHCH₂CH₃

• CH₃CH₂CH=CHCH₂CH₃

Answer explanation

P and Q are two hydrocarbons with empirical formulae CH₂ and CH respectively. Under the same conditions, P decolourises bromine water while Q does not. Which mechanisms are correct about the reactions of P and Q?

• P : Electrophilic addition

  Q : Electrophilic substitution

• P : Electrophilic addition

  Q : Nucleophilic substitution

• P : Nucleophilic substitution

  Q : Nucleophilic substitution

• P : Electrophilic

  substitution

  Q : Electrophilic substitution

Answer explanation

P decolourises bromine water, showing that it is an alkene. Alkenes undergo electrophilic addition.

Q does not decolourise bromine water and yet it is unsaturated (ratio of C : H = 1 : 1). Q may be benzene, C₆H₆. Benzene undergoes electrophilic substitution.

The reaction scheme for the production of a tertiary alcohol, C₆H₁₄O from X is shown in the diagram. X could be

• 2,

  2−dimethylpropanol

• 2-methyl-2-butanol

• 1-pentanol

• 3-pentanol

Answer explanation

Phenol and benzoic acid are soluble in NaOH. However, phenol (a weak acid) is insoluble in sodium carbonate, but benzoic acid do.

A hydrocarbon is prepared by passing alcohol X, C₄H₁₀O, over hot aluminnium oxide. The hydrocarbon reacts with chlorine to form 1, 2-dichloro-2-methylpropane. Which is true about alcohol X?

• Alcohol X is chiral.

• Alcohol X is a secondary alcohol.

• Alcohol X is 2-methyl-2-propanol.

• Alcohol X has two structural isomers.

Answer explanation

Oxidation of X to form an intermediate (let us called it X). X then reacts with a Grignard's reagent to produce a tertiary alcohol. Since only a ketone reacts with Grignard's reagent to produce a tertiary alcohol, X has to be a secondary alcohol.

P reacts with an aqueous solution of sodium hydroxide and with nitrous acid at 0°C without releasing any gas. What is the structural formula of P?

• 
• 
• 
• 

Answer explanation

Reaction with NaOH indicates that P is acidic. P could be a phenol or carboxylic acid. Reaction with cold —NH₂ group bonded directly to the benzene ring.

A carbonyl compound reacts with the alkaline solution of copper (Ⅱ) complex and alkaline solution of potassium manganate (Ⅶ). Which statement is correct about the reactions?

• Colourless products are found.

• The carbonyl compound is ketone.

• The reactions are nucleophilic addition.

• The reactions form an organic salt as one of the products.

Answer explanation

An alkaline solution of copper (Ⅱ) complex is also known as Fehling's solution. Since the carbonyl compound reacts with Fehling's solution (a reddish brown precipitate of copper (Ⅰ) oxide), it is an aldehyde.

RCHO + 2Cu²⁺ + 5OH^- → RCOO^- + Cu₂O + 3H₂O

Oxidation by alkaline KMnO₄ produces the carboxylate ions.

RCHO + [O] + OH^- → RCOO^- + H₂O

Both reactions are redox where the aldehyde is oxidised to the carboxylate ion.