Properties  of Alcohols

• Physical Properties of Alcohols

Lower alcohols are colourless, volatile, toxic, inflammable liquids with burning taste and spirituous odour. Above C₁₂, alcohols are solid.

Alcohols, in contrast, contain the very polar -OH group. In particular, this group contains hydrogen attached to the very electronegative element, oxygen, and therefore permits hydrogen bonding. The physical properties show the effect of this hydrogen bonding.

Let us look first at boiling points. Among hydrocarbons the factors that determine boiling point seems to be chiefly molecular weight and shape; this is to be expected of molecules that are held together chiefly by vander Waals forces. Alcohols, too, show increase in boiling point with increasing carbon number, and decrease in boiling point with branching. But the unusual thing about alcohols is that they boil at so high temperature, much higher than hydrocarbons of the same molecular weight, and higher, even, than many the compounds of considerable polarity.

Alcohols, like water, are associated liquids: Their abnormally high boiling points are due to the greater energy needed to break the hydrogen bonds that hold the molecules together. Although ethers and aldehydes contain oxygen, they contain hydrogen that is bonded only to carbon; these hydrogens are not positive enough to bond appreciably with oxygen.

The solubility behaviour of alcohols also reflects their ability to form hydrogen bonds. In sharp contrast to hydrocarbons, the lower alcohols are miscible with water. Since alcohol molecules are held together by the same sort of intermolecular forces as water molecules, there can be mixing of the two kinds of molecules: the energy required to break a hydrogen bond between two water molecules or two alcohol molecules is provided by formation of a hydrogen bond between a water molecule and an alcohol molecule.

This is true, however, only for the lower alcohols, where the -OH group constitutes a large portion of the molecule. A long aliphatic chain with a small -OH group at one end is mostly like alkane, and its physical properties show this. For practical purposes we consider that the borderline between solubility and insolubility in water occurs at about four to five carbon atoms for normal primary alcohols.

Polyhydroxy alcohols provide more than one site per molecule for hydrogen bonding, and their properties reflect this. The simplest glycol, ethylene glycol, boils at 197^o. The lower glycols are miscible with water, and those containing as many as seven carbon atoms show appreciable solubility in water.

• Chemical Properties of Alcohols

Reactions involving H atom of -OH groups

All such reactions involving replacement of H atom of -OH gp represent Bronsted acid nature of alcohols. The Bronsted acid nature shows the order :

methyl alcohols > 1^o > 2^o > 3^o alcohol

The tendency to attract electron pair of O-H bond by O atom decreases by the +ve I.E. of CH3 gps as it intensifies the partial -ve charge on oxygen atom & thus tendency to release H atom decreases in tertiary alcohols.

Following reactions of this category are noticed.

Action of active metals :  Strong electropositive metals like Na, K, Ca & Mg react with alcohols forming alkoxides with the liberation of H₂ gas.

R-OH + Na → R-ONa + 1/2H₂.

Action of carboxylic acids or esterification : Alcohols on reacting with anhydrous acids in presence of conc. H₂SO₄ form esters. Concentrated H₂SO₄ acts as catalyst as well as dehydrating agent. Esterification eraction is..

R-OH + HOOCR'   R'COOR + H₂O                                                                              

Action of acetyl chloride and acetic anhydride or acetylation :Replacement  of H atom by acetyl gp. The H atom of alcohol is replaced by acetyl group CH₃CO-)

Tertiary alcohols are not acylated, however, they react with acetyl chloride to give alkyl chlorides and in some cases alkenes.

Action of Grignard reagent :

All these reactions show the reactivity order P.A. > S.A. T.Alcohol

Reactions Involving Replacement of -OH groups of alcohols  

The -OH gp of alcohol has two lone pair of electrons on O atom & thus behaves as Lewis base. The Lewis base nature of alcohols show the order.

 T.A. > S.A. > P. Alcohol

or      3^o > 2^o > 1^o > methyl alcohol

K, tertiary alcohols the partial -ve charge on oxygen atom is intensified due to + I.E. of CH₃ gps. This increases the tendency of oxygen atom to donate electron pair or to show cleavage of C-O bond.
Action of inorganic acids : Forms esters with inorganic acids 

Action of phosphorus halides :

3ROH + PCI₃ → 3RCI + H₃PO₃

ROH + PCI₅ → RCI + POCI₃ + HCI

Similar reactions are obtained by using P + Br₂ and P + I₂ for alkyl bromide and iodide preparation because of less stable nature of PBr₃ & PI₃

Action of SOCI₂ :

R-OH + SOCI₂   R-CI + SO₂ + HCI
 

Action of Halogen Acids

Alcohols react readily with hydrogen halides to yield alkyl halides and water. The reaction is carried out either by passing the dry hydrogen halide gas into the alcohol, or by heating the alcohol with the concentrated aqueous acid. Sometimes hydrogen bromide is generated in the presence of the alcohol by reaction between sulfuric acid and sodium bromide.

The less reactive of the hydrogen halides, HCl, requires the presence of zinc chloride for reaction with primary and secondary alcohols; on the other hand, the very reactive tert-butyl alcohol is converted to the chloride by simply being shaken with concentrated hydrochloric acid at room temperature. For example:

Note :

1. If alcohols are heated with conc. HI and red P, alkanes are formed R-OH + 2HI   R-H + H₂O + I₂

2. For getting bromo and iodo derivatives from alcohols, a mixture of KBr + H₂SO₄and KI + H₂SO₄ respectively can be used.

3. Primary alcohols follow S_N2 mechanism during replacement of OH group by halogen atom
   

4. Secondary and tertiary alcohols follow S_N1 mechanism during replacement of OH group by halogen atom.
   

The reactivity order for halogen acids is : HI > HBr > HCI

Let us list some of the facts that are known about the reaction between alcohols and hydrogen halides.

1. The reaction is catalyzed by acids -  Even though the aqueous hydrogen halides are themselves strong acids, the presence of additional sulfuric acid speeds up the formation of halides.

2. Rearrangement of the alkyl group occurs, except with most primary alcohols - The alkyl group in the halide does not always have the same structure as the alkyl group in the parent alcohol
   For example:
   
   We see that the halogen does not always become attached to the carbon that originally held the hydroxyl (the first example); even the carbon skeleton may be different from that of the starting material (the second example).
   On the other hand, n-propyl and n-butyl alcohols, most primary alcohols give high yields of primary halides without rearrangement.

3. The order of reactivity of alcohols toward HX is allyl, benzyl >3^o>2^o>1^o<CH₃. Reactivity decreases through most of the series (and this order is the basis of the Lucas test), passes through a minimum at 1^o, and rises again at CH₃.

Action of NH₃ :   

Action of H₂S :

All these reactions (a) to (f) show the reactivity order T.A. > S.A. > P. Alcohol.
 

Reactions Involving Complete Molecule of Alcohols 

Dehydration : 

Dehydration (removal of H₂O) of alcohols yields alkene or ether depending up-on the experimental conditions. The dehydration of alcohol favour the order T.A. > S.A. > P. Alcohol. The action of various dehydrating agents used are given below. 

We recognize this mechanism as an example of E₁ elimination with the protonated alcohol as substrate. We can account, in a general way, for the contrast between alcohols and alkyl halides, which mostly undergo elimination by the E₂ mechanism. Since the alcohol must be protonated to provide a reasonably good leaving group, H₂O, dehydration requires an acidic medium. But for E₂ elimination we need a fairly strong base to attack the substrate without waiting for it to dissociate into carbonium ions.

Mechanism of Dehydration of Alcohols - According to the commonly accepted mechanism, we remember, dehydration involves (1) formation of the protonated alcohol, ROH2+, (2)  its slow dissociation into a carbonium ion, and (3) fast expulsion of a hydrogen ion from the carbonium ion to form an alkene. Acid is required to convert the alcohol into the protonated alcohol, which dissociates-by loss of the weakly basic water molecule-much more easily than the alcohol itself.

Reactivity of Alcohols toward Dehydration:

-We know that the rate of elimination depends greatly upon the rate of formation of the carbonium ion, which in turn depends upon its stability.

We know how to estimate the stability of a carbonium ion, on the basis of inductive effects hyperconjugative effect and resonance. Because of the electron-releasing inductive effect of alkyl groups, stability and hence rate of formation of the simple alkyl cations follows the sequence 3o>2o>1o.

We know that because of resonance stabilization the benzyl cation should be an extremely stable ion, and so we are not surprised to find that an alcohol such as 1-phenylethanol (like a tertiary alcohol) undergoes dehydration extremely rapidly.

Orientation - We know that expulsion of the hydrogen ion takes place in such a way as to favor the formation of the more stable alkene. We can estimate the relative stability of an alkene on the basis of the number of alkyl groups attached to the doubly-bonded carbons, and on the basis of conjugation with a benzene ring or with another carbon-carbon double bond. It is understandable, then, that sec-butyl alcohol yields chiefly 2-butene, and 1-phenyl-2-propanol yields only 1-phenylpropene.

Rearrangement - Finally, we know that a carbonium ion can rearrange, and that this rearrangement seems to occur whenever a 1,2-shift of hydrogen or alkyl group can form a more stable carbonium ion. In all this we must not lose sight of the fact that the rates of formation of carbonium ions and of alkenes depend chiefly upon the stabilities of the transition states leading to their formation. A more stable carbonium ion is formed faster because the factors-inductive effects and resonance-that disperse the charge of a carbonium ion tend also to disperse the developing positive charge of an incipient carbonium ion in the transition state. In the same way, the factors that stabilize an alkene-conjugation of hyperconjugation, or perhaps change in hybridization-tend to stabilize the developing double bond in the transition state.

Note :

1. Normally secondary & tertiary alcohols give alkene as main product on dehydration.

2. The mechanism of dehydration of alcohols giving alkene involves protonation of the alcohol followed by loss of a water molecule and a proton.
   

3. The mechanism of dehydration of alcohol giving ether involves protonation of alcohol. An unprotonated molecule of alcohol then combines with the protonated molecule of alcohol losing water molecule. The oxonium ion formed gives ether by a loss of a proton.
   

Action of halogens 

1. Halogens oxidize primary and secondary alcohols to aldehydes and ketones respectively.

2. After oxidation, the halogens also show substitution at  α-carbon atom of products.
   

Note : Tertiary alcohols are not oxidised by halogens and thus tertiary butyl alcohol does not give iodoform if heated with halogens and alkali.

Acetal formation :

Haloform reaction : Ethyl alcohol and all alkanol-2 (secondary alcohols only) undergo haloform reaction on heating with halogen and alkali. Follow haloform reaction.

Uses :

(i) In beverage industry

(ii) As antiseptic - spirit

(iii) As fuel - spirit lamp

(iv) As solvent for medicines

(v)  As an antifreeze; glycol and glycerols are used with water to form an antifreeze mixture to cool down the radiators.

• Alcohols as Acids

We have seen that an alcohol, acting as a base, can accept a hydrogen ion to form the protonated alcohol, ROH₂⁺. Let us now turn to reactions in which an alcohol, acting as an acid, loses a hydrogen ion to form the alkoxide ion, RO^-.

Since an alcohol contains hydrogen bonded to the very electronegative element oxygen, we would expect it to show appreciable acidity. The polarity of the O-H bond should facilitate the separation of the relatively positive hydrogen as the ion; viewed differently, electronegative oxygen should readily accommodate the negative charge of the electrons left behind.

The acidity of alcohols is shown by their reaction with active metals to form hydrogen gas, and by their ability to displace the weakly acidic hydrocarbons from their salts (e.g., Grignard reagents):

ROH + Na → RO^-Na⁺  + 1/2H₂

 ROH + R'MgX  →   R'H + Mg(OR)X

            Stronger              Weaker

              acid                     acid

With the possible exception of methanol, they are weaker acids than water, but stronger acids than acetylene or ammonia:

RO^-Na⁺ + H-OH  →   Na⁺OH^- + RO-H

Stronger   Stronger     Weaker   Weaker

 base        acid            base       acid

As before, these relative acidities are determined by displacement. We may expand our series of acidities and basicities, then, to the following:

 Relative acidities  :      H₂O > ROH > HC º CH > NH₃ > RH

Relative basicities :     OH^- < OR^- < HC º C^- < NH₂^- < R^-

Not only does the alkyl group make an alcohol less acidic than water, but more the number of alkyl group, the less acidic the alcohol: methanol is the stronger acid and tertiary alcohols are the weakest.

• Oxidation of Alcohols

The number of oxidizing agents available to the organic chemist is growing at a tremendous rate. As with all synthetic methods, emphasis is on the development of highly selective reagents, which will operate on only one functional group in a complex molecule, and leave the other functional groups untouched. Of the many reagents that can be used to oxidize alcohols, we can consider only the most common ones, those containing Mn(VII) and Cr(VI).

Primary alcohols can be oxidized to carboxylic acids. RCOOH, usually by heating with aqueous KMnO₄. When reaction is complete, the aqueous solution of the soluble potassium salt of the carboxylic acid is filtered from MnO₂, and the acid is liberated by the addition of a stronger mineral acid.

Primary alcohols can be oxidized to aldehydes, RCHO, by the use of K₂Cr₂O₇. Since, as we shall see aldehydes are themselves readily oxidized to acids, the aldehyde must be removed from the reaction mixture by special techniques before it is oxidized further.

Secondary alcohols are oxidized to ketones, R₂CO, by chromic acid in a form selected for the job at hand: aqueous K₂Cr₂O₇, CrO₃ in glacial acetic acid, CrO₃

in pyridine, etc. Hot permanganate also oxidizes secondary alcohols; it is seldom used for the synthesis of ketones, however, since oxidation tends to go past the ketone stage, with breaking of carbon-carbon bonds.

With no hydrogen attached to the carbonyl carbon, tertiary alcohols are not oxidized at all under alkaline conditions. If acid is present, they are rapidly dehydrated to alkenes, which are then oxidized. Tertiary alcohols are most difficult to oxidise among the three classes of alcohols.

• Important Facts about Alcohols

 1. Toxicity of alcohol 

Ethyl alcohol < isopropyl alcohol < methyl alcohol

or   Grain alcohol < rubbing alcohol < wood alcohol

2.    Alcohol content in drinks :

Whiskey » Rum » Brandy > Undistilled   >    Beer     >     Cidar

3.   Rectified spirit : The content obtained after rectification of fermented liquid.

4.   Absolute alcohol :

(i)   Ethyl alcohol 99.5 to 100%

(ii)   On distillation ethanol forms an azeotropic mixture containing 95.6% ethanol. 100% ethanol can be obtained either by keeping rectified spirit with lime for about 24 hous & then distilling it (lab method) or by adding small amount of C₆H₆ & then distilling it (Industrial method)

5.   Power alcohol : Rectified spirit + C₆H₆ + Petrol, for generation of power.

6.   Methylated spirit :

(i)   Methanol + Pyridine + mineral naptha + Rectified spirit.

(ii)   These are added to make sure that it will not be used for bevarages. This process is known as denaturation & alcohol is known as denatured spirit.

Type :    

(a)   Mineralized methylated spirit : 90% Rectified spirit + 10%    methanol or pyridine or naphta

(b)   Industrial methylated spirit : 95% Rectified spirit, 5% methanol.

7.     Proof spirit :

(i)     The % of alcohol is expressed in terms of proof spirit for tax lavied on it.

(ii)     Proof spirit is ethanol (57.1%) + water

(iii)   The weakest possible alcohol % which allows gun powder to catch fire is known as proof spirit.

(iv)   10^o under proof means 100 volume of sample containing as 90 volume of proof spirit.

(v)    10^o over proof means 100 volume of sample containing as 110 volume of proof spirit.

(vi)   The determination of strength of alcohol is known as alcoholimetry.

(vii)  A sample is called over under proof, as it is stronger or weaker than proof spirit.

• Distinction Test

Distinction test for primary, secondary and tertiary alcohols

1. By oxidation :

 The nature of the oxidation products of alcohols depends upon the nature of alcohol. The oxidizing agent used commonly are acidified K₂Cr₂O₇, alkaline or acidified KMnO₄or dilute HNO₃.

Note :

In case of primary alcohol, aldehyde and aci both have same no. of C atoms as parent alcohols have.

In case of secondary alcohols, ketone has same no. of carbon atoms but acid has lees carbon atom than alcohol and ketone.

Tertiary alcohols are not oxidized in neutral or alkaline medium, however under drastic oxidation it gives ketone and acid both having lesser number of carbon atoms that the parent alcohol.

 In case of oxidation of unsymmetrical ketones, the splitting of the ketonic chain takes place according to Popff's rule, that the carbonyl group always remains attached to a smaller alkyl gp.      

2. By catalytic dehydrogenation :

The dehydrogenation of alcohol vapours passed over reduced Cu at 300^oC gives different products depending upon the nature of alcohols.

3. By Victor Meyer's test :  Alcohols are subjected to a series of operations given below. The final colour of solution indicates the nature of alcohol. 



Addition of NaOH simply increases the dissociation of nitrolic acid to increase the concentration of red coloured anions.

4. Lucas reagent test : This test is based on the reactivity order of alcohols to replace -OH gp by halogen atom i.e. T.A. > S.A. > P.A. The alcohols are allowed to react with Lucas reagent (i.e. conc. HCI and ZnCI_2 anhydrous) and following observations are noticed.

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