`text(Reaction with Alkali Metals :)`

Active metals (`Na`, `K`, `Mg`, `Al` etc.) when treated with alcohols give hydrogen gas. In this reaction, order of reactivity of alcohols is `CH_3OH > 1^o > 2^o > 3o.` This reaction exhibits acidic character of alcohols.

`RO -H +Na -> RO^(-) Na^(+) +1/2 H_2`

In `-OH` group of alcohols, oxygen is more electronegative than hydrogen, this results in polarization of `O-H` bond due to which acidic nature arises in alcohols. Reaction of active metals with alcohols shows that alcohols are acidic in nature.

`undersettext(Stronger base)[RO^(-)Na^(+)] + undersettext(Stronger acid)(HOH) -> undersettext(Weaker base)(NaOH) + undersettext(Weaker acid)(ROH)`

The order of acidity for some compounds is

`H_2O > ROH > HC equiv CH > NH_3 > RH`

The order of basicity is

`R^(-) > NH_2^(-) > HC equiv C^(-) > OR^(-) > OH^(-)`

The above order is based on the reactions of alcohols with other species.

`C_2H_5OH + Na -> C_2H_5O^(⊖)Na^(oplus) + 1/2 H_2 uparrow`

`HC equiv C^(-)Na^(+) + RO -H -> HC equiv CH +RO^(-)Na^(+)`

`text(Esterification :)` A direct reaction between a carboxylic acid and alcohol under the catalytic effect of sulphuric acid yields an ester. This is a reversible reaction and is known as the `" Fischer esterification ".` See fig.

Polarization of the `C-O` bond makes the carbon atom partially positive, so this carbon would be susceptible to the nucleophilic attack and if it were not for the fact that `OH^-` is a strong base and they are very poor leaving group. Protonation of the alcohol converts a poor leaving group `(OH^(-))` into a good one. It also make the carbon atom even more positive (because `-OH_2^(+)` is more electron withdrawing than `- OH` ) and therefore, even more susceptible to nucleophilic attack. Now, nucleophilic substitution reactions are possible in alcohols according to the given mechanism.

`text(Reaction with H-X :)`

For example, See fig.1.

In `S_N 1` reactions of alcohol (when `R` group is `3^o`), `R` may rearrange. Primary alcohols and methanol apparently react through a mechanism that we recognize as an `S_N2` type.

With `HBr`, alcohols produce alkyl bromide. See fig.2.

or for stable carbocation `R^+`, racemic mixture will be obtained for optically active alcohol by `S_N 1` mechanism.

But `3`- pentanol reacts with `HBr` to produce `2`- and `3`- bromopentane derivatives. See fig.3.

In `S_N i` reaction, retention of configuration is observed & mechanism operates through intimate ion-pair formation.

With `PBr_3`, `PBr_5` also alkyl bromide results with no rearrangement. Sometimes alcoholic `OH` is made good leaving group by converting `O-H` into `-OTs` group. See fig.4.

For the compound like `CH_3CH(OH)CH=CH_2` double bond shifts and resultant bromide is a mixture as shown below. See fig.5.


`text(Reaction with)` `PX_3` `text(AND)` `PX_5` :

Alcohols react with `PX_3` and `PX_5` to yield alkyl halides `(PX_3 = PBr_3, PI_3)`

`underset[1^o text(or) 2^o](3R - OH) + PBr_3 -> 3R - Br + H_3PO_3`

`R- OH + PCl_5 -> R- Cl + POCl_3 + HCl`

For example, See fig.6.

`text(Reaction with) SOCl_2 :`

Alcohols react with thionyl chloride in presence of pyridine to give alkyl chloride with inverted configuration, while in absence of pyridine, an alkyl chloride with retention of configuration is obtained via `S_N i` mechanism.

`R- OH + SOCl_2 -> R- Cl + SO_2 + HCl`

`text(Acid Catalysed Dehydration of Alcohols :)`

Alcohols in presence of dilute acid undergo dehydration forming alkenes. The reaction proceeds by `E_1` mechanism. See fig.7.

Dehydration of cyclic alcohol is accompanied by expansion in the above reaction. Stability of the ring is given by Baeyer strain theory, according to which the stability order of the rings is `6 > 7, 5 > 8, 9 > > 4 > 3 `.

Alcohols with atleast one hydrogen atom on `1^o` and `2^(o)` carbinol are oxidised to carbonyl compounds (aldehydes and ketones). PCC (Mixture of pyridine, `HCl` and `CrO_3`) oxidises `1^o` alcohol to aldehydes but `K_2Cr_2O_7` or `KMnO_4` in acid converts `1^o` alcohol directly to carboxylic acids. Under mild conditions, `3^o` alcohols are not oxidised.

`text[Jone's reagent (chromic acid in aqueous acetone solution) :]` This is a sufficiently mild oxidising agent, so that it oxidises alcohols without oxidising or rearranging double bonds. `MnO_2` can oxidise `1^o` allylic or `1^o` benzylic alcohols selectively into aldehydes.

For example,

`CH_3- CH=CH- CH_2OH undersettext(in acetone)overset(H_2CrO_4)-> CH_3-CH=CH-CH=O`

See fig.

The most convenient reagent for the oxidation of alcohols is `8N` chromic acid in sulphuric acid (Jones reagent). Two millimoles of this reagent oxidizes `3` millimoles of monohydric alcohol according to the equation :

`3R_2CHOH + 2H_2CrO_4 -> 3R_2C=O + 2Cr(OH)_3 + 2H_2O`

The function of sulphuric acid is to prevent complex formation of `Cr`(`VI`) with its reduced form `Cr` (`III`) to a salt having much less oxidation potential. This ensures that all the `Cr`(`VI`) is used in oxidation, which thus becomes rapid and complete. Acetone is the usual solvent used at ice bath temperature. Alcohols having double or triple bonds in the molecule can be selectively oxidized to ketones in good yields. For example, See fig.

The most likely mechanism for the oxidation of alcohols by Jones reagents has been shown to be

`R_2CH- OH + Cr^(6+) -> R_2C= O + Cr^(4+) + 2H^(+)`

`Cr^(4+) + Cr^(6+) -> 2Cr^(5+)`

`ul[3Cr^(5+) + 2R_2CH -OH -> 2R_2C= O + 2Cr^(3+) + 4H^(+)]`

`3R_2CH - OH + 2Cr^(6+) -> 3R_2C= O + 2Cr^(3+) + 4H^(+)`

Thus, apparently, `3` moles of alcohols react with `2` moles of `Cr`(`VI`) to give `3` moles of ketone and `2` moles of `Cr`(`III`). It is clear, however, that only one mole of the alcohol is oxidized directly by `Cr`(`VI`) and the other two part in the oxidation with `Cr` (`V`).

Lead tetraacetate or periodic acid are commonly used for the cleavage of 1, 2- glycols.

The former reagent is used in anhydrous solvent, whereas the later in organic solvent. Periodic acid is more selective and readily cleaves 1, 2- glycols at room temperature. But cleavage of an `alpha`- hydroxy ketone or acid by this reagent even at higher temperature is slow. Lead tetraacetate, however, oxidizes a hydroxyketone or acid as well as 1, 2- glycols more easily. This is explained on the basis of a five membered cyclic intermediate. See fig.1.

A 1, 2- glycol need not necessarily be cis to undergo cleavage with lead tetraacetate.

For instance, trans-9, 10-decalindiol undergoes cleavage to cyclodecane-1, 6-dione. See fig.2.

Upon treatment with periodic acid, `HIO_4`, compounds containing two or more `-OH` or `C=O` groups attached to adjacent carbon atoms undergo oxidation with cleavage of carbon-carbon bonds.

For example : See fig.3.

The oxidation is particularly useful in determination of structure. Qualitatively, oxidation by `HIO_4` is indicated by formation of a white precipitate `(AgIO_3)` upon addition of silver nitrate. Since the reaction is usually quantitative, valuable information is given by the nature and amounts of the products, and by the quantity of periodic acid consumed.

The general reaction is represented as : See fig.1.

The `CH_3` carbon is lost as `CHX_3` and the remaining part exists as acid salt, which can be acidified to liberate free acid.

The structural feature essential in the compound to show haloform reaction is that any of the following moieties should be present in the molecule attached to some electron - withdrawing group or electron donating group by `+I` only. See fig.2.

or any other grouping that can be converted to any of the above moieties.

The mechanism of the reaction can be outlined as :

The reaction has `3` important steps. Step I is the oxidation, caused by mild oxidizing agent (hypo halite ion). The second step is base - promoted halogenation and the third step is cleavage of `C-C` bond. See fig.3.

Some of the compounds which responds positively to iodoform test are

`CH_3CH_2OH` (only primary alcohol)

See fig.4.

The compounds that respond negatively to iodoform test are : See fig.5.

The chemical properties of an alcohol, `ROH`, are determined by its functional group, `-OH`, the hydroxyl group. Reactions of an alcohol can involve the breaking of either of two bonds : the `C-OH` bond, with removal of the `-OH` group; or the `O--H` bond, with removal of `-H`. Either kind of reaction can involve substitution, in which a group replaces the `-OH` or `-H`, or elimination, in which a double bond is formed.