How to predict whether primary, secondary, and tertiary alcohol uner goes halogenation?

Question

We did a lab and we had to predict whether
1-butanol (primary) , 2-butanol (secondary), and 2-methyl-2-propanol (tertiary) undergo halogenation. We mixed those 3 alcohols with concentrated HCl (aq)...

How do you predict whether this goes under halogenation?
And what does cloudiness means if it occurs? Because the only one that cloudiness appeared was for the tertiary alcohol mixed with the HCl (aq)

Thank you =)

Answer 1

The first answer is somewhat correct but still does not adequately explain why the difference between primary, secondary and tertiary alcohols.

The reaction of a tertiary alcohol with acid (either HCl or HBr) takes place readily at 0℃ via an SN1 reaction (remember SN1 means, substitution, nucleophilic, unimolelcular). When a lone pair on the oxygen atom snags the acidic proton from the acid, water becomes the leaving group, and leaves behind a tertiary carbocation. Knowing carbocation stability rules, tertiary is more stable than secondary, than primary than methyl. This means it is easy for the tertiary carbocation to exist as a separate species after the leaving group (water ) leaves. Now the nucleophilic chlorine atom which is left from the acid can come in and attack the carbocation, forming your tertiary alkyl halide.

The reactions of secondary and primary alcohols with either SOCl2 or PBr3 take place via an SN2 mechanism (Remember, the secondary, primary and methyl carbocations are too poor to exist in solution as a separate stable species, so SN2 is the only other mechanistic possibility here). Hydroxide itself is too poor a leaving group to be displaced by nucleophiles in SN2 reactions - but the reaction with one of these two reagents converts the hydroxide into much better leaving groups, namely -OSOCl or -OPBr2 - these are readly expelled during the backside attack of either the nucleophilic Cl- or Br-.

So as explained, only the tertiary alcohol you worked witih in lab would undergo conversion to the corresponding alkyl halide. The others would be unreactive with HCl.

Email me if you have more questions about this.

Answer 2

You were apparently using the Lucas reagent, which is anhydrous zinc chloride dissolved in concentrated hydrochloric acid. The reagent reacts best with tertiary alcohols, which is why you saw the "cloudiness." The cloudiness was the appearance of an immiscible tert-butyl chloride in the aqueous acid. 2-Butanol should also have given a reaction, but slower. 1-Butanol should have been slowest of all. The reason is the high stability of the tert-butyl carbocation.

Answer 3

Bromination Of Alcohol

Answer 4

when an alcohol is halogenized it will want to form the most stable comformation, for that reason primary alcohols will not undergo this reaction, but tertiary will thus forming the most substituted product

Answer 5

why tertiary alkyl halide are best in sn1 reaction and primary alkyl halide in sn2