In presence of aqueous sodium or potassium hydroxide or moist silver oxide.

`R - X+ KOH(aq) -> R - OH + KX`

Reaction will follow either `S_N 1` or `S_N 2` mechanism.

In presence of reducing agents like `H_2` in `Ni// Pt//Pd` or `[H]` or `LiAlH_4` or `NaBH_4`. Esters can however only be reduced by `LiAlH_4` or `[H]`.

`R - CH O -> RC H_2OH` (`1^^o` alcohol)

Aldehydes can be reduced to primary alcohols and ketones to secondary alcohols, either by catalytic hydrogenation or by the use of chemical reducing agents like lithium aluminium hydride `(LiAlH_4), H_2//Ni, B_2H_6//THF` etc.

The commonly used reagent for the reduction of aldehydes and ketones are `LiAlH_4` or `NaBH_4`.

The reaction proceeds by successive transfer of hydride ions (` H^-`) from boron or aluminium to four different carbonyl carbons. As all four of the hydrides are transferred, there are distinct reducing agent used in every step of reduction.

In the first step, the reducing agent is `AlH_4^-` (I), while in the second step, the reducing agent is `RCH_2OAlH_3^-` ( II).

In the same manner, species `[RCH_2O]_2AlH_2^-` (species Ill ) and `[RCH_2O]_3AlH^-` (species IV) will be obtained. Reduction with `NaBH_4` is usually done in water or in alcohol which are used as the solvent The solvent destroys the intermediate alkoxide ion and yields alcohol. If water or an alcohol is not used as the solvent, aqueous acid can be added after the reduction to convert the alkoxide to alcohol.

The mechanism for reduction with lithium aluminium hydride is very similar to that of `NaBH_4`. As `LiAlH_4` violently reacts with water and other polar protic solvents to give molecular hydrogen, reduction with `LiAlH_4` are done in aprotic solvents like anhydrous ether.

Ethyl acetate is added to decompose the aluminium complex. But reduction with `NaBH_4` can be carried out in water or ethanol solution.

Aldehydes are reduced to primary alcohols by both `LiAlH_4` and `NaBH_4` while these reducing agents reduce ketones to secondary alcohols.

`LiAlH_4` can reduce all functional groups of column `A` into Column `B`. Generally, it is unable to reduce double bond, which are in conjugation with `text()_(//)^(\\)C=O` group the double bond is reduced when a phenyl group is attached to the `b` - carbon.

For example,

`C_6H_5 CH=CH CHO overset ( LiAlH_4) (->) C_6H_5CH_2CH_2CH_2- OH`

Fig 4a Showing can reduce all the above groups of column 'A' into groups of column 'B' except, -`COOH ` and -`COOR. NaBH_4` is able to reduce the double bonds, which are in conjugation with `text()_(//)^(\\)C=O` group.

It can reduce all the above groups of column `A` into groups of column `B`. It can also reduce double and triple bond present in carbonyl compounds irrespective of the position in carbon chain whether it is in conjugation with carbonyl group or not.

`CH_3- CH=CH-CHO overset (H_2//Ni) (->) CH_3CH_2CH_2CH_2-OH`

`CH_2=CH-CH_2-CHO overset (H_2//Ni) (->) CH_3CH_2CH_2CH_2-OH`

It can reduce all above groups of column `A` into groups of column `B` except, `- COCl`.

Lithium aluminum hydride, `LiAlH_4`, is one of the few reagents that can reduce an acid to an alcohol; the initial product is an alkoxide from which the alcohol is liberated by hydrolysis:

`underset(1^o text(alcohal))(4RCOOH) + 3LiAlH_4 -> 4RCH_2OH`

Because of the excellent yield it gives, `LiAlH_4` is widely used in the laboratory for the reduction of not only acids but many other classes of compounds.

Like many organic compounds, esters can be reduced in two ways:

(a) by catalytic hydrogenation using molecular hydrogen, or
(b) chemical reduction

In either case, the ester is cleaved to yield (in addition to the alcohol or phenol from which it was derived) a primary alcohol corresponding to the acid portion of the ester.

`RCOOR' undersettext(Reduction) rightarrow RCH_2OH + R'OH`

When a solution of an alkyl halide in dry ethyl ether, `(C_2H_5)_2O`, is allowed to stand over turnings of metallic magnesium, a vigorous reaction takes place. The solution turns cloudy, begins to boil and the magnesium metal gradually disappears. The resulting solution is known as Grignard reagent. It is one of the most useful and versatile reagents known to the organic chemists.

`undersettext(Alkyl halide)(RX) + Mg oversettext(Dry ether ) (->) undersettext( Alkyl magnesium halide)(RMg X)`



The Grignard reagent has the general formula `RMgX`, and the general name alkyl magnesium halide. The carbon magnesium bond is covalent but highly polar, with carbon pulling away electrons from electropositive magnesium but the magnesium - halogen bond is essentially ionic, ` overset (-) (R) overset (+)(Mg) X`

Since magnesium becomes bonded to the same carbon that previously held halogen, the alkyl group remains intact during the preparation of the reagent. Thus, `n` - propyl chloride yields n - propyl magnesium chloride and isopropyl chloride yields isopropyl magnesium chloride.

`undersettext(n-Propyl chloride)(CH_3CH_2CH_2Cl) + Mg oversettext (Dry ether) (->) undersettext(
n - Propyl magnesium chloride)(CH_3CH_2CH_2MgCl)`


`undersettext(Isopropyl chloride)((CH_3)_2CHCl) + Mg oversettext(Dry ether) (->) undersettext( Isopropyl magnesium chloride)((CH_3)_2CHMgCl)`



The Grignard reagent belongs to a class of compounds called organometallic compounds, in which carbon is bonded to a metal like lithium, potassium, sodium, zinc, mercury, lead, thallium or to almost any metal known. Each kind of organometallic compound has its own set of properties and its particular uses depend on these. But whatever the metal, it is less electronegative than carbon and the carbon - metal bond is always highly polar. Although the organic group is not a full - fledged carbanion but has considerable carbanionic characters. Thus, organometallic compounds can serve as a source of carbon bearing negative charge.

The Grignard reagent has the formula `RMgX` and is prepared by the reaction of metallic magnesium with the appropriate organic halide.
This halide can be alkyl `(1^o, 2^o, 3^o)`, allylic, aryl alkyl (e.g. benzyl), or aryl (phenyl) or substituted phenyl. The halogen may be -`Cl`, -`Br` or -`I`, (Aryl magnesium chlorides must be made in the cyclic ether tetrahydrofuran instead of ethyl ether). Aldehydes and ketones resemble each other closely in most of their reactions. The carbonyl group is also unsaturated and like the carbon - carbon bond, it also undergoes addition. One of the typical reaction is cis addition of the Grignard reagent.

The electrons of the carbonyl double bond hold together atoms of different electronegativity, thus, the electrons are not equally shared, the mobile `p` - cloud is pulled strongly towards the more electronegative atom, oxygen. The addition of an unsymmetrical reagent happens such that the nucleophilic (basic) portion attaches itself to carbon and the electrophilic (acidic) portion attaches itself to oxygen.

The carbon - magnesium bond of the Grignard reagent is a highly polar bond, carbon being negative relative to electropositive magnesium. When Grignard reagent is added to carbonyl compounds, the organic group attaches to carbon and magnesium to oxygen.

The product is a magnesium salt of the weakly acidic alcohol and is easily converted into the alcohol by the addition of the stronger acid, water. The `Mg(OH)X` thus formed is a gelatinous material, which forms coating over carbonyl compound, thus dilute mineral acid
`(HCI, H_2SO_4)` is commonly used instead of water, so that water - soluble magnesium salts are formed.

The type of alcohol that is obtained from a Grignard synthesis depends upon the type of carbonyl compound used. Formaldehyde `(HCHO)` yields primary alcohols, other aldehydes `(RCHO)` yield secondary alcohols and ketones `(R_2CO)` yield tertiary alcohols. The number of hydrogens attached to the carbonyl carbon defines the carbonyl compound as formaldehyde, higher aldehydes or ketone. The carbonyl carbon is the one that finally bears the `- OH` group in the product and the number of hydrogen defines the alcohol as pnmary, secondary, or tertiary. For example, Show in Fig 2

Reactivity order of the substrates with a Grignard reagent is

Active -`H text(compounds) > ``- CHO > rangleC=O > -COCl > -CO_2R`

`> - CH_2X`.

Sometimes, Grignard reagent does not react with compounds containing functional group normally capable of undergoing reaction. Generally, branching of the carbon chain near the functional group prevents the reaction. For example, methyl magnesium bromide or iodide does not react with hexamethyl acetone `(CH_3)_3 C COC(CH_3)_3`. It has also been found that if Grignard reagent contains large alkyl groups, reaction may be prevented. For example, isopropyl methyl acetone reacts with methyl magnesium iodide but not with `t -` butyl magnesium iodide. In other cases, abnormal reaction may take place. For example, when isopropyl magnesium bromide is added to diisopropyl ketone, the expected tertiary alcohol is not formed, instead the secondary alcohol, di - isopropyl carbinol is obtained resulting from the reduction of the ketone.

`(CH_3)_2CHCOCH(CH_3)_2 overset {(CH_3)_2CHMgBr} { ->}`

`(CH_3)_2CHCH(OH)CH(CH_3)_2 + CH_3CH=CH_2`

a , b - unsaturated carbonyl compounds adds on Grignard reagents at the `1, 2` or `1, 4` - positions.

`1, 2 -` addition : As shown in Fig. 3a

`1, 4 -` addition : As shown in Fig. 3b

The reaction of dihalides of the type `Br(CH)_nBr` with magnesium depends on the value of `n`. For `n = 1`, no Grignard reagent is formed. For example,

(IMAGE REACTION)

When `n^3 4,` the Grignard reagent can be possibly made with dibromo compound i.e, `Br(CH_2)_nBr.`

Alkyl lithium compounds can be prepared by direct displacement, the chlorides give best yields. For example,

`Bu-Cl + 2 Li underset (hexan e) overset (N_2) (->) Bu -Li + Li Cl`

Other organolithium compounds can be prepared by the halogen - metal exchange with the butyl lithium compound.

`Bu-Li + R-X -> R- Li + Bu -X`

Organolithium compounds behave like the Grignard reagent but the lithium compounds are usually more reactive and the yield of the product is often better. Because of their sensitivity to oxygen and to water, reaction with lithium alkyls is best carried out in an atmosphere of dry nitrogen. Alkyl lithium reacts with `CO_2` to give carboxylate salt.

`R-Li + CO_2 -> R- COO^(-) Li^+`

Aliphatic primary amines on treatment with nitrous acid give primary alcohols.

`RCH_2NH_2 + HNO_2 -> RCH_2OH + N_2 + H_2O`

`CH_2 = CHR + underset (text (Concentrated )) (H_2SO_4) underset (H_2O) oversettext (Boil) (->) CH_3-CH(R)OH`

`R = R + CO +H_2 underset ([Co(CO)_4]_2) overset (text( high temperature)) (->)R-R-CHO overset (H_2//Ni) (->) R-R-CH_2OH`

`undersettext(Sucrose)(C_12H_22O_11) +H_2O oversettext(Invertase)rightarrow undersettext(Glucose)(C_6H_12O_6) + undersettext(Fructose)(C_6H_12O_6) oversettext(Zymase) rightarrow 2C_2H_5OH + 2CO_2`