12th Class Chemistry Chapter 10 Alkyl Halides Short Questions Answer

Chemistry short QA

12th Class Chemistry Chapter 10 Alkyl Halides Short Questions Answer

1.What are alkyl halides?Give their general formula and name two monohaloalkanes?
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 CnH2n+`1-X.CH3CI,Br,C3H7I are mone halo alkanes.
2.What are primary secondary and tertiary alkyl halides?Give example of each?
3.What are the mecessary conditions to convert alcohols into alkyl halides by using halogen acids?
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
4.SOCI2 is the best reagent to get alkyl halides from alcohols.Which solvent is necessary to complete this reaction?
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.
5.Convert ethanol to get their respective halides by using PCI3 and PI5?
6.Which two factors are responsible for reactivity of alkyl halides as compared to alknes?
i) Bond dissociation energy (ii) polarity
The bond dissociation energy of C-X bond is in the order.
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.
7.Define and give one example of each.i) Nucleophile ii) Leaving group?
Nucleophile:Those species which are electron effcient and they donate their electron pairs.
Leaving group or nucleophile:
The halogen ion which breaks the bond from a-carbon is called leaving group or nucleophile.
8.Give the general pattern of the reaction of SN2-mexhanism.What is its rate expression?Give examples?
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
9.Give the general pattern of reaction of SN1 mechanism.Why tertiary alkyl halides give this mechanism?
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.
10.What is the role of steric hinderance to decide about SN1 and SN2 mechanisms?
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.
11.What is the role of stability of carbonium ion for determining SN1 or SN2 mechanism?
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.
12.How do you compare SN1 and SN2 mechanism?
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.
13.What are elimination reactions?Give examples of E1 and E2.?
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.
14.How tetraethyl lead can be prepared from an alkyl halide?
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.
15.How the nature of alkyl group changes when alkyl halide is converted to grignanr’s reagent?
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.
16.What is Grignard’s reagent?Why dry ether is necessary for the preparation of Grignard’s reagent?
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.
17.How do we get alkyl nitrles from Grignard’s reagent?
18.Give the mechanism for the addition of Grignard’s reagent at CO2 ti give carboxylic acids?
19.Give the mechanism for the reaction of formaldehyde with ethyl magnesium bromide?
20.What is the mechanism for the reaction of Grignard’s reagent with acetaldehyde?
21.Show the mechanism for the reaction of acetone with Grignard’s reagent?
22.How primary alcohols are produced when ethene epoxide is reacted with Grignard’s reagent?
23.How compounds with active – H react with G.R to give alkanes?
HOR + R’ – Mg – X ……………… R’ – H + Mg(OH) X
R – OH + R’ – MgX …………….. R’ – H + MgX
24.What is an excellent method to prepare alkyl halides?
Alkyl chlorides and alkyl bromides are reacted with Nal to get R-I
CH3CH2CI + Nal……………..CH3CH2I + NaCI
CH3CH2Br+ Nal……………..CH3CH2I+ NaBr

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