Test: JEE Main 35 Year PYQs- Alcohols, Phenols & Ethers - JEE MCQ

The dehydration of alcohol to form alkene occurs in following three step. Step (1) is initiation step.
Step (1) Formation of protonated alcohol.

Step (2) Formation of carbocation

Step (3)Elimination of a proton to form ethene

3-methyl pentanol-3 will be dehydrated most readily since it produces tertiary carbonium ion as intermediate.

Pyridiminum chloro-chromate (PCC) is specific for the conversion.

Methyl vinyl ether under anhydrous condition at room temperature undergoes addition reaction.

Only those alcohols which contain –CHOHCH₃ group undergo haloform reaction. Among the given options only (b) contain this group, hence undergo haloform
reaction.

NOTE : OH group activates the benzene nucleus and

NOTE : This is Riemer-Tiemann reaction and the electrophile is dichlorocarbene.

Tertiary alcohols react fastest with conc. HCl and anhydrous ZnCl₂ (lucas reagent) as its mechanism proceeds through the formation of stable tertiary carbocation.
Mechanism

Whenever dehydration can produce two different alkenes, major product is formed according to Saytzeff rule i.e. more substituted alkene (alkene having lesser
number of hydrogen atoms on the two doubly bonded carbon atoms) is the major product.
Such reactions which can produce two or more structural isomers but one of them in greater amounts than the other are called regioselective ; in case a reaction is 100% regioselective, it is termed as regiospecific.
In addition to being regioselective, alcohol
dehydrations are stereoselective (a reaction in which a single starting material can yield two or more stereoisomeric products, but gives one of them in greater amount than any other).

Electron withdrawing substituents like –NO₂, Cl increase the acidity of phenol while electron releasing substituents like – CH₃, – OCH₃ decreases acidity. hence the correct order of acidity will be

Further (– I) NO₂ > (–I) Cl and (+ I) CH₃ > (+I) OCH₃

Tertiary alchols reacts fastest with lucas reagnet as the rate of reaction is directly proportional to the stability of carbocation formed in the reaction. Since most stable
3° carbocation is formed in the reaction hence it will react fastest further tetriary alcohols appears to react by S_N1 mechanism.

Among 20 naturally occuring amino acids "Cysteine" has '– SH' or thiol functional group.
⇒ General formula of amino acid 

⇒ Value of R = –CH₂–SH in Cysteine.

N – bromosuccinimide results into bromination at allylic and benzylic positions

When tert -alkyl halides are used in Williamson synthesis elimination occurs rather than substitution resulting into formation of alkene. Here alkoxide ion abstract one of the b-hydrogen atom along with acting
as a nucleophile.