This set of Organic Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “Reaction of Alcohols”.
1. Which one of the following compound is obtained by the oxidation of primary alcohol with nascent oxygen?
Explanation: Oxidation of alcohols to aldehydes is partial oxidation; aldehydes are further oxidized to carboxylic acids. Conditions required for making aldehydes are heat and distillation. In aldehyde formation, the temperature of the reaction should be kept above the boiling point of the aldehyde and below the boiling point of the alcohol.
2. Which one of the following compound is obtained by the oxidation of secondary alcohols by [O]?
Explanation: Ketone compound is obtained by the oxidation of secondary alcohols by nascent oxygen atom [O].
3. Secondary alcohols on catalytic dehydrogenation by Cu-Ni couple gives ___________
c) Carboxylic acid
Explanation: When the vapours of a secondary alcohol are passed over copper heated at 573K, the corresponding ketone is formed.
4. The reaction of carboxylic acids with alcohols catalysed by conc. H2SO4 is called __________
Explanation: When a carboxylic acid is treated with an alcohol and an acid catalyst, an ester is formed (along with water). This reaction is called the esterification.
5. Which one is the correct order of reactivity of different types of alcohol towards hydrogen halide?
a) 1° alcohol > 2° alcohol > 3° alcohol
b) 2° alcohol > 1° alcohol > 3° alcohol
c) 3° alcohol > 1° alcohol > 2° alcohol
d) 3° alcohol > 2° alcohol > 1° alcohol
Explanation: Reaction of HX with alcohol depends on the degree of carbonation generated due to heterocyclic cleavage of HX bond. If alcohol is primary alcohol then reaction will proceed through Sn2 for tertiary alcohol it will proceed with Sn1. For secondary alcohol it can go through both path (Sn1 or Sn2). Sn1 being ionic in nature takes place very fast while Sn2 is molecular in nature reacts slowly. Hence, we can sum up above, Rate of reaction is 3° alcohol > 2° alcohol > 1° alcohol.
6. The dehydration of alcohols is an example of ________
a) Bimolecular elimination/E2 reaction
b) SN2 reaction
c) SN1 reaction
d) Unimolecular elimination/E1 reaction
Explanation: Alcohol upon reaction with protic acids tends to lose a molecule of water to form alkenes. These reactions are known as dehydration of alcohols. Dehydration of alcohols is an example of Unimolecular elimination reaction.
7. Among the following, which is least acidic?
d) p- chlorophenol
Explanation: O-cresol is least acidic because Presence of electron withdrawing groups at ortho and para position increases the acidic strength, whereas electron releasing or donating groups at ortho and para position decreases acidic strength. In cresol (ortho or para cresol), methyl group is +I group, so it destabilises the anion formed after ionisation, hence cresol is less acidic than carbolic acid (phenol).
8. Which of the following alcohols would be most soluble in water?
Explanation: Propanol is the most soluble in water among given alcohols, as solubility of alcohols in water decreases with increase in molecular mass.
Explanation: Thionyl bromide will react with secondary alcohols and give a reasonable percent yield of the bromide.
10. The major reason that phenol is a better Bronsted acid than cyclohexanol is ____________
a) it is a better proton donor
b) the cyclohexyl group is an electron donating group by induction, which destabilizes the anion formed in the reaction
c) phenol is able to stabilize the anion formed in the reaction by resonance
d) the phenyl group is an electron withdrawing group by induction, which stabilizes the anion formed in the reaction
Explanation: Whenever resonance exists, it usually is a very important factor to be considered in decisions about structure and reactivity correlations and in many cases, it is the most important factor. In this case there is delocalization of the electrons into the benzene ring.
Sanfoundry Global Education & Learning Series – Organic Chemistry.
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