Chemical Properties of Alcohols Preparation and Properties
The reactions shown by alcohols may be divided into following four types :
→ Reactions involving both the alkyl and hydroxyl groups.
→ Due to presence of electron pair oñ oxygen atom of hydroxyl group.
→ Reaactions Involving the cleavage of Bond of Hydroxyl Group
→ Order of ease for the cleavage of O—H bond in alcohols is : Primary Alcohol > Secondary Alcohol > Tertiary Alcohol
→ Reaction with Active Metals : Acidic nature Alcohols react with strong electropositive metalš like
Na, K, Mg, Al, Zn etc. to liberate hydrogen gas and form metal alkoxide.
→ This reaction shows that alcohols are acidic in nature. The acidic nature of alcohols is due to electronegative oxygen atom, which withdraws the shared electron pair between O—H towards itself. As a result, O—H bond becomes weak and loses a proton. However, alcohols are weak acids (Ka = 1 × 10-1010 – 10-18) even weaker than water (Kw = 1 × 10-14).
→ In Alcohol O—H group is attached with alkyl group. Alkyl group have +I effect. The alkyl group releases electrons of C—O bond towards oxygen atom and increases electrorr density around it and therefore polarity of O—H bond decreases.
→ As a result, the tendency of oxygen to withdraw electrons in O—H bond towards itself decreases and therefore, the release of proton (H+) becomes difficult. In short we can say that due to +1 effect of alkyl groúp alcohols are weaker acids than water.
Comparision of Acidic Strength of Primary, Secondary and Tertiary Alcohols :
→ In primary alcohols, only one alkyl group is present on the carbon to which —OH group is attached (R—OH). In secondary alcohol two alkyl groups are attached or
→ Alkyl group have electron donating effect. This effect is maximum in tertiary alcohols. Due to this, O—H bond is least polar and release of proton is difficult. Therefore order of acidity in isomeric alcohols is as follows : Primary Alcohol > Secondary Alcohol > Tertiary Alcohol
The basic strength of their alkoxide ions is in reverse order.
→In the presence of strong acids alcohols act as bonsted bases. Hence, they accept proton from strong acids to form protonated alcohols.
→ Reaction with metal hydrIdes : Alcohols react with metal hydrides to form metal alkoxides and H2 gas.
R — OH + NaH → R — ONa + H2 ↑
→ Reaction with carboxyllc acids : (Esterification) Alcohols react. with carboxylic acids in the presence of conc. H2SO4 or dry HCl gas to form esters.
→ This reaction is known as esterification. The reaction is reversible and takes place slowly. As the size of bulky groups in carboxylic acids and alcohols increases, the rate of esterification decreases. Therefore order of reactivity of alcohols and carboxylic acids will be as follows :
→ Alcohols are versatile compounds. They behave both as nucleophiles as well as electrophiles.
They behave as nucleophiles in the reactions in which the bond between O—H is broken as shown below :
→ They behave as eectrophiles in which the bond between C—O is broken. These reactions are carried out in the presence of acids to form protonated alcohols. Protonated alcohols react as electrophiles.
→ Acylation : When alcohol is treated with acid chloride or anhydride, the hydrogen atom of—OH group is replaced by acyl group (RCO—) forming esters. This reaction is called acylation.
→ If the acid chloride and anhydridé used are acetyl chloride (CH3COCl) and acetic anhydride (CH3CO)2O then this reaction is known as acetylation because here acetyl group (CH3CO—) is attached.
→ Acetylation of alcohols is done in the presence of bases like pyridine or dimethyl aniline. ‘l’he process of acetylation is used to protect the hydroxyl groups in various reactions. The acetyl derivatives of alcohols are readily hydrolysed to regenerate the alcohols.
→ Reaction with Grignard Reagent : Alcohols have active hydrogen atoms. They react with Grignard reagents to form alkanes.
→ Reactions Involving cleavage of bond of alkanol.
→ Alcohols undergo a number of reactions involving the cleavage of bond. In these reactions, the order of reactivity of alcohols is:
→ Tertiary Alcohol > Secondary Alcohol > Primary Alcohol It is important to note that esterification involves the cleavage of the O—H bond of alcohol. This has been established by isotopic tracer techniques. If the reaction is carried out with alcohol having isotopic oxygen O18.
(Which can be easily traced because of its radioactive properties), the resulting ester contains this isotope of oxygen.
→ If the C—O bond of alcohol would have cleaved during esterfflcation, O18 would have gone to resulting water.
→ Reaction with Hydrogen Halides : Alcohols react with hydrogen halides to form haloalkanes and water. The order of reactivity of hydrogen halides is
HI >HBr >HCl
→ Mechanism : The reactivity of the above reaction can be easily understood in terms of its mechanism. Primary alcohols react by SN 2 mechanism whereas secondary and tertiary alcohols react by SN 1 mechanism as:
→ The reaction of 20 or 30 alcohols by SN 1 mechanism, quite often gives rearranged products which are different from the expected products. This occurs due to 1, 2-hydride shift or 1, 2-methyl shift.
→ This is because of rearrangement of the carbocation as shown below:
→ A hydride shift transforms the 2° carbocation into a more stable 3° carbocation and attack by bromide ion gives the observed major product.
→ Similarly, neopentyl alcohol, (CH3)3CCH2OH reacts with HBr to give 2-Bromo-2-methyl butane as rearranged product.
→ Reaction with Phosphorus Trihalide : Alcohols react with phosphorus trithalide (PCl3, PBr3, PI3) to form haloalkanes. Phosphorus trthalide is prepared by the reaction between phosphorus and halogen.
→ PBr3 and PI3 are generally prepared in situ by the reaction between phosphorus and bromine or iodine.
→ Reaction with Phosphorus Pentachioride Alcohols react with phosphorus pentachloride (PCl5) to form chioroalkane.
→ Reaction with Thionyl Chloride : On treatment with thionyl chloride alcohols form chioroalkanes. This reaction is carried out in the presence of pyridine. This is the best method for the preparation of chioroalkanes. The other byproducts SO2 and HCl formed are gaseous and can easily be separated.
→ Reaction with Ammonia : Alcohols react with ammonia in the presence of dry ZnCl2 at 573 K temperature, to form alkanamine.
→ Reduction : In the presence of red phosphorus and HI alkanols are reduced to alkanes.
→ Reactions due to both alkyl and hydroxyl groups.
→ Reaction with conc. H2SO4 (Dehydration ReactIon) : When alcohols are heated with conc.
→ H2SO4, at different temperatures different products arc obtained. At 413 K in the presence of excess of alcohol, ether is formed.
→ When alcohols are heated in excess of cone. H2SO4 in between 433-443 K temeprature, one water molecule is eliminated from alcohol and alkene is formed. This reaction is known s dehydration reaction of alcohol.
Mechanism : The dehydration of alcohols takes place in three steps:
→ First Step : Due to the presence of two lone pair of electrons on oxygen atom, alcohols behave as weak bases. Therefore they react with strong mineral acids like H2SO4 to form oxonium salt of protonated alcohol.
→ Second Step : The protonated alcohol loses a water molecule to form a carbocation. This step is rate determining step and slow step.
→ Third Step : The carbocation then eliminates a proton to form alkene.
→ Order of reactivity of primary, secondary and tertiary alcohols towards dehydration reaction is as follows:
→ Tertiary alcohol > secondary alcohol > Primary alcohol As in this reaction intermediate carbocation is formed and tertiary carbocations is most stable therefore tertiary alcohol easily gives dehydration reaction. Order of stability of primary, secondary and tertiary carbocations is as follóws:
3° > 2° > 1°
→ If there is possibility of formation of more than one alkene, then that alkene is formed as major product which has more substituents on double bonded carbon.
Example.
→ This is according to Saytzeff’s rule. The carbocation formed during the reaction, get rearranged to more stable carbocation and the final product is formed from this rearranged carbocation.
→ Dehydrogenation : The dehydrogenation of alcohols is achieved with reduced copper at 573 K. Primary alcohols form alkanal
→ Dehycirogenation of secondary alcohols form alkanone.
→ When vapours of tertiary alcohols are passed on copper powder, dehydration occurs and formation of alkene takes place.
→ Oxidation : By the oxidation of primary, secondary and tertiary alcohols different products are obtained. The oxidation is carried out by neutral, alkaline or acidified KMnO4 , acidified K2Cr2O4 or dil HNO3.
→ The oxidation of alcohols involves the formation of a carbon-oxygen double bond with cleavage of an O—H and C—H bonds.
→ This type of cleavage and formation of bonds occur in oxidation reaction. These reactions are also called dehydrogenation reactions because these involves loss of hydrogen from alcohol.
→ Oxidation of Primary Alcohol : A primary alcohol is oxidised to form first on aldehyde and further oxidised to form carboxylic acid. Both the aldebyde and the acid formed contain the same number of carbon atoms as the starting alcohol.
→ Oxidation of Secondary Alcohol : Secondary alcohol is oxidised to form a ketone. which have the same number of carbon atoms as the starting alcohol.
→ Ketones are not easily oxidised further but when they react with oxidising agent for a longer time they form carboxylic acid which have lesser number of carbon atoms.
→ This oxidation can be stopped at the ketone lavel by using chromic anhydride (Cr03).
→ Oxidation of tertiary alcohols : In neutral or basic medium tertiary alcohols do not show oxidation reaction. When treated with acidic oxidising agents they form ketone and mixture of alkanoic acids. Products contains lesser number of carbon atoms than the starting alcohols.
→ Reactions Due to Presence of Lone Pair of electrons on Oxygen Atom of Hydroxyl Group : Due to the presence of two lone pair on oxygen atom of hydroxyl group they behave as Lewis bases and form oxonium salt with mineral acids.
→ Ascent of Carbon Chain In Alcohols:
→ Descent of Carbon Chain in Alcohol
