Properties of Alcohol

PHYSICAL PROPERTIES

Solubility

Alcohols are soluble in water due to formation of H – bonding between water & them. As the molecular mass increases, the alkyl group become larger which resists the formation of

H – bonds with water molecules and hence the solubility goes an decreasing.

Boiling Point

Intermolecular H – bonding is present between alcohol molecules. This makes high boiling point.

Amongst the isomeric alcohols, the order of boiling point is 1º > 2º > 3º alcohol.

CHEMICAL PROPERTIES

Chemical properties of alcohols can be discussed under following categories:

(A) Reaction involving breaking of carbon – oxygen bond.

(B) Reaction involving breaking of oxygen – hydrogen bond.

(C) Oxidation of alcohols.

(D) Dehydrogenation of alcohols.

(E) Some miscellaneous reactions of monohydric alcohol.

(A) Reaction involving breaking of carbon – oxygen bond

     Order of reactivity of alcohol. 3º > 2º > 1º

(i)  SN reaction

(ii) Dehydration of alcohol

    ꃞhydration of alcohol to give alkene.

     (a)  ꃞhydrating agents are

           ਌onc H₂SO₄/$\Delta$, KHSO₄/$\Delta$, H₃PO₄/$\Delta$, Anhyd Al₂O₃/$\Delta$, Anhyd PCl₅/$\Delta$, Anhyd ZnCl₂/$\Delta$, BF₃/$\Delta$, P₂O₅/$\Delta$.

     (b)   Reactivity of alcohols. (Ease of dehydration)

            ਃº > 2º > 1º

     (c)   Product formation always takes place by saytzeff rule. 

$\bullet$ Alcohols on acetylation gives acetyl derivative which on pyrolytic elimination always gives Hofmann product. 

Mechanism in presence of acidic medium

E₁ mechanism: follow saytzeff’s rule.

(B) Reactions due to breaking of oxygen hydrogen bond. (Reactions due to acidic character of alcohols)

     (a)  ਊlcohols are acidic in nature because hydrogen is present on electro negative oxygen atom.

     (b)  ਊlcohol is weaker acid 

acidity $\propto$ stability of acid anions.

            ꂬidity of 1º > 2º > 3º

            ਊlcohols give following reactions due to breaking of oxygen – hydrogen bond.

(i)        Reaction with metal

M = 1^st group metal.

     M = Al, Mg, Zn

     $3R-OH+Al\to {\left(RO\right)}_{3}Al+\frac{3}{2}{H}_2$

                                     ਊluminium alkoxide

(ii) Esterification (With carboxylic acid)  

It is reversible acid catalysed reaction. It follow SN₁ mechanism.

Increasing the size of alkyl group on alcohol part decreases the nucleophilic character because steric hindrance increases.

.

     Order of reactivity of alcohols CH₃OH > 1º alc > 2º alc > 3º alc

(iii) Ester formation with proton acid having –OH group: to give inorganic ester 

(iv) Alkylation of Alcohol

              R-O-H\xrightarrow[\begin{smallmatrix} or \\#xA0C{{H}_{3}}I/{{K}_{2}}C{{O}_{3}} \end{smallmatrix}]{{{\left( C{{H}_{3}} \right)}_{2}}S{{O}_{4}}/NaOH}R-O-C{{H}_{3}}

     Methylation is mainly used for determination of hydroxyl groups in an unknown compound.

     $No.ofhydroxylgroups=\frac{Molecularweightofmethylatedetherprodcued-molecularweightofreac\tan t}{14}$

(C) Oxidation of alcohol

           Oxidation of alcohol is dehydrogenation reaction which is 1, 2 – elimination reaction.

So oxidation of alcohol $\propto$ numbers of $\alpha$ - hydrogen atom.

     (a) With mild oxidising agents:

           Like

           (i)  X₂

           (ii) Fenton reagent [FeSO₄/H₂O₂]

           (iii)      Jones reagent / CH_૓COCH₃ [CrO₃/dil. BaSO₄]

           (iv)      K₂Cr₂O₇/H⁺ cold

Note:

     PCC (Pyridinium chloro chromate) is a selective reagent which converts 1º alc to aldehyde.

     (b) With strong oxidising agent

          Oxidising agents are

          (i)        $KMn{O}_4/O{H}^{-}/\Delta$                    (ii) $KMn{O}_4/H^{+}/\Delta$

                      (iii)      $K_{2}C{r}_2{O}_7/H^{+}/\Delta$             (iv)$Conc.HN{O}_3/\Delta$         $\underset{\left(ncarbon\right)}{RC{H}_{2}OH}\stackrel{\left[O\right]}{\to }\underset{\left(ncarbon\right)}{\mathrm{RCOOH}}$

(D) Dehydrogenation with Cu/573K or Ag/573K

     (a)ਁº alcohol $\to$ aldehyde

          $R-C{H}_{2}OH\stackrel{Cu/573K}{\to }RCHO$

     (b) ਂº alcohol $\to$ ketone

          $R-CHOH-R\stackrel{Cu/573K}{\to }R-CO-R$

           (c)      ਃº alc $\to$ undergo dehydration to form alkene.

Reduction

(E) Miscellaneous reactions of mono hydric alcohol

     (i)  Methylation with CH₂N₂ in presence of BF₃.

          $R-O-H\underset{B{F}_3}{\overset{C{H}_2{N}_2}{\to }}R-O-C{H}_3+N_2$

     (ii) Haloform reaction

                     ਎thyl alcohol and 2º methyl alcohol gives haloform reaction.

Distinguishing 1º, 2º, 3º alcohol

PERIODATE OXIDATION

Compounds that have hydroxyl group on adjacent atoms undergo oxidation cleavage when they are treated with aq. Periodic acid (HIO₄). The reaction breaks carbon carbon bonds and produced carbonyl compounds (aldehyde, ketones or acids) 

It takes place through a cyclic intermediate. 

Other examples

This oxidation is useful in determination of structure.

Illustration 1.  Write the products of the reaction of t-butyl alcohol with PBr₃, conc. H₂SO₄, CH₃COCl, Na, CH₃MgBr, Na₂Cr₂O₇/H₂SO₄.

Solution:                   ${\left(C{H}_3\right)}_{3}CBr,{\left(C{H}_3\right)}_{3}C=C{H}_2{\left(C{H}_3\right)}_{3}COCOC{H}_3,{\left(C{H}_3\right)}_{3}C{O}^{-}N{a}^{+},C{H}_4,noreaction$.

PINACOL – PINACOLONE REARRANGEMENT

Action of H₂SO₄ on 1, 2 diols.

Ditertiory – 1, 2 diols convert in to ketones on treatment wit H₂SO₄.

Mechanism

Migratory preference of the group

Migration depends on the stability of Transition state.

In general migration of C₆H₅ > alkyl

Illustration 2.  List three methods with chemical equations for the preparation of alcohols from alkenes.

Solution:       Hydration, hydroboration and oxymercuration – demercuration of alkenes.

Hydration:

Hydroboration

Oxymercuration – demercuration