12th Class Chemistry Chapter 10 Alkyl Halides Short Questions Answer

Those organic compounds which are derived from the hydrocarbons by the replacement of one or more hydrogen atoms by halogen atoms are called alkylhalides.They are represented by R-X and the general formula is C_nH_2n+`1-X.CH₃CI,Br,C₃H₇I are mone halo alkanes.

Alcohols are reacted with halogen acids in the presence of ZnCI₂ or H₂SO₄ to get alkyl halides,
R-OH+H…………ZnCI₂ or H₂SO₄……….. R-X+H₂O

Alcohols react with SOCI₂ to give alkyl cholrides in the presence of pyridine.HCI and SO₂ are gaseous products which are evolved automatically and pure alkyl chloride is obtained.So this is one of the best methods.

3CH₃CH₂OH+PCI₃……………….3CH₃CH₂CI+H₃PO₃
CH₃CH₂OH+PI₅…………………..CH₃CH₂I+POI₃+HI

i) Bond dissociation energy (ii) polarity
The bond dissociation energy of C-X bond is in the order.
C-F>C-CI>Br>C-I
It means C-I bond can be broken very easily so idides are very reactive substances.In alkanes there is no polarity.All these bonds are polar.

Nucleophile:Those species which are electron effcient and they donate their electron pairs.
Examples:
CI^–,Br^–,I^–,OH^–,CN^–,NO₂^–,HS^–,C₂H₅O^–,CH₃O^–,SCN^–,H₂O,NH₃,NH₂^–,etc.
Leaving group or nucleophile:
The halogen ion which breaks the bond from a-carbon is called leaving group or nucleophile.

SN₂ reaction takes place in one step in which the attack of the attacking nucleophile  and the leaving of the leaving nuclephile takes place simultaneously.Its  rate depends upon the concentration of substrate and the attacking nucleophile.
R-X + Nu^–…………… R-Nu + X^–
Rate = k[R-X][Nu]
CH₃Br+OH^–……………… CH₃OH+Br^–
CH₃CH₂-l+OH^–……………. CH₃CH₂OH+l^–

In SN₁ mechanism the reaction takes place in two steps.
In the first step C – X bond breaks and in the second step the attacking  nucleophile attacks  the carbonium  ion.Tertiary carbonium ion is more stable than secondary and primary so tertiary alkyl halides can give this mechanism.
R  X…. slow….. R⁺ + X^–
R⁺+Y^–…..Fast…. R-Y.

If big groups are attached at the a-carbon of alkyl halides the backside attack for  the attacking nucleophile becomes difficult .For this  reason SN₂ mechanism is favoured rather than SN₁.

Greater the stability of the carbonium  ion greater the possibility for two step mechanism which is SN₁.These alkyl halides which can provide unstable carbonium ion give SN₂ mechanism which is a single step reaction.

SN₁ mechanism is in two steps.While SN₂ is  in a single step.The rate of SN₁ is  controlled by only  one molecule.Tertiary  alkyl halides mostly give SN₁ while primary give SN₂.

E₁ atands for elimination  unimolecular and tertiary alkyl halide give this reaction to give alkenes.E₂ is elimination of bimoleculars in which  primary alkyl  halides gives alkenes in one step.

E₁ stands for elimination unimolecular and tertiary alkyl halide give this reaction to give alkenes.E₂ is elimination of bimoleculars in which primary alkyl halides gives alkenes in one step.

In alkyl halides the alkyl group is  electrophilic  in character because the a-carbon  bears the partial positive  charge.When it is converted to Grignard’s reagent the a-carbon  develops the partial negative  charge and the alkyl group becomes nnucleophilic in character.The nuture of C-Mg bond changes and it makes G.R a very reactive substance.

The alkyl magnesium halide is called G.R.They are prepared by reacting an alkanes aryl halide with Mg in dry ether.If ether is wet the water molecules react with Grignard’s reagent to give alkanes and so Grignard’s reagent is destroyed.Actually water is an active hydrogen compound for Grignard’s reagent.

HOR + R’ – Mg – X ……………… R’ – H + Mg(OH) X
R – OH + R’ – MgX …………….. R’ – H + MgX

Alkyl chlorides and alkyl bromides are reacted with Nal to get R-I
CH₃CH₂CI + Nal……………..CH₃CH₂I + NaCI
CH₃CH₂Br+ Nal……………..CH₃CH₂I+ NaBr