Chemical Reactions of Primary Alcohols

A primary alcohol undergoes three broad classes of reaction: reaction of the acidic $-\text{OH}$

hydrogen itself, replacement of the whole $-\text{OH}$ group by another group, and oxidation at the

carbon bearing $-\text{OH}$.

Reaction with active metals. Like water, an alcohol's $-\text{OH}$ hydrogen is weakly acidic

and is displaced by reactive metals such as sodium, evolving hydrogen gas and forming a metal

alkoxide:

The vigorous fizzing this produces (much gentler than sodium's reaction with water, since an

alcohol is a weaker acid than water) is itself a simple qualitative test confirming the presence of

an $-\text{OH}$ group.

Replacement of $-\text{OH}$ by halogen. A primary alcohol reacts with a hydrogen halide (best

with $\text{HCl}$/$\text{ZnCl}_2$ or, more reliably and in higher yield, with phosphorus

halides -- $\text{PCl}_3$, $\text{PCl}_5$ or $\text{SOCl}_2$ (thionyl chloride)) to replace the

$-\text{OH}$ group entirely with a halogen, giving the corresponding primary haloalkane, e.g.

$\text{CH}_3\text{CH}_2\text{OH} + \text{SOCl}_2 \rightarrow \text{CH}_3\text{CH}_2\text{Cl} + \text{SO}_2 + \text{HCl}$. Thionyl chloride is often preferred synthetically because both

by-products ($\text{SO}_2$ and HCl) are gases that simply escape, leaving a clean product without

the aqueous work-up a simple $\text{HX}$ reaction would need.

Oxidation. A primary alcohol's carbon bears one hydrogen directly on the

$-\text{OH}$-substituted carbon, and oxidation removes this hydrogen (and the $-\text{OH}$'s own

hydrogen) stepwise. A strong oxidising agent such as acidified potassium dichromate

($\text{K}_2\text{Cr}_2\text{O}_7$/$\text{H}_2\text{SO}_4$) or potassium permanganate

($\text{KMnO}_4$) oxidises a primary alcohol first to an aldehyde, and then, since the

reaction does not readily stop there under these vigorous conditions, straight on to a

carboxylic acid:

To isolate the aldehyde as the actual product, a milder, selective oxidant such as pyridinium

chlorochromate (PCC) is used instead, which oxidises the primary alcohol only as far as the

aldehyde and does not oxidise it further. (A secondary alcohol, by the same logic, is oxidised

only as far as a ketone, since a ketone carbon has no remaining C--H to lose; a tertiary alcohol

has no hydrogen at all on its $-\text{OH}$ carbon and so resists oxidation by these reagents under

normal conditions.)

Esterification. A primary (or any) alcohol reacts with a carboxylic acid, in the presence of a …