12th Class Chemistry Chapter 8 Aliphatic Hydrocarbons Short Questions Answer

Those organic compounds which are made up of carbon and hydrogen only are known as hydrocarbons.When we keep in view their cyclic or open chain structures.there are two types. i)Acylic ethane ethene and ethyne are acyclic hydrocarbons. ii) Cyclic:Cyclopropane cyclobutane cyclohexane and benzene ets are cyclic hydrocarbons.

General formulas are C_nH_2n+2 for alkanes C_nH_2n for alkenes and C_nH_2n-2 for alkynes.

Alkanes do not have any functional group.All the carbon atoms are sp³ hybridized.C-C bonds are o in nature.There bonds are chemically inert.There are no  multiple bonds.Due to these resoons alkanes have least tendency to react with acids bases oxidizing agents and reducing agents.Alkennes have carbon carbon  double bonds.The electrons of the π bond are diffused alkenes give the addition reactions while alkanes give subsitution reactions.

Branched chain alkanes have low boiling points as compared to linear chain alkanes.

A saturated gydrocarbon has sufficient hydrogens in it  but in unsaturated  hydrocarbons double and truple bonds are there.They give addition reaction reactions.While saturated gydrocarbons give subsititutions reactions.

a) We can do the quantitative measurement of unsaturation in the compound and find the number of double bonds in it. b) Oils are converted into vegetable ghee by the saturation process.

Alkyl halides upon reduction give corresponding alkanes.The important reducing  agents are Zn+HCI,C₂H₅OH+Na,HI,P+HI and Zn –Cu couple +HCI.

Clemmensen’s reduction and Wolf-Kishner reduction convert the carbonyl groups of ketones and aldehdes into- CH₂– group.So ketones and aldehdes are converted into corresponding alkanes.They are carried out in the presence of Zn/Hg + HCI and N₂H₄+ KOH respectively.

The active hydrogen compounds are HOH,ROH,RCOOH etc.They give their hydrogen to the alkyl group of Grignard’sreagent to give  alkanes and the carbon atoms do not increase.
HO H+R-    Mg – X ……………..R-H +Mg      X
OH

Lkanes are saturated compounds having all sigma bonds.Sigma bonds are very stable.So alkanes are less reactive than alkenes and alkynes.These families have weaker π bonds in them.

The reaction of alkanes with oxygen are highly exothermic.The amount of heat evolved which is called the heat of combustion of alkanes is used for many purposes.So they act as fuels.

When higher alkanes say butane and pentane are reacted with conc.HNO₃ at 400-500 ᵒC the nitro-derivatives are obtained by the substitution of hydrogen of alkanes by nitro groups.Due to high temperature carbon carbon bond is  also broken.But in case of CH₄,no C-C bond breaks and following reaction happens.
CH₄+HNO₃……………….CH₃NO₂+H₂O

The reaction happens in the presence of U.V light diffused sunlight  or at 300-400ᵒC temperature.Molecule of CI₂ is broken up into its free radicals which further produce the free redicals.In this way chloroderivatives of methane are produced.

Alkenes gives oily liquids when treated with halognes like CI₂ and Br₂.The word olefiant means oil forming.

The alkenes having only one double bond like ethene are called monoenes.The alkenes having two double bonds like CH₂=CH =CH₂are called dienes.the alkenes having more than two double bonds are called polyenes.

When we remove halogens from α carbon and one of the hydrogens from B-carbon a double bond is created between C-α and C-B.In this way hydrogens and halogens are removed and alkene is produced.
CH₂-CH₂– CI………………..CH₂=CH₂+H₂O
alc.KOH
H

The dehydration of alchols completes through the formation of carbonium ions.Tertiary alcohols give a stable carbonium ions than taht of secondary than primary.So this order determines the reactivity of alcohols.

When the sodium salt of succinic acid is electrolysed  in solution decarboxylation takes place and the double  bond is created between  two carbon atoms.Ethene is collected  at the anode while hydrogen is collected at the cathode.
2CH₃COONa+H₂O………….. C₂H₆+2CO₂+NaOH+H₂

When alkynes are reacted with hydrogen in the presence of Lindlar catalyst then cis alkenes are produced.When they are treated with sodium in liquid ammonia then trans alkenes are produced at-33ᵒC.

The electrons of the π bonds are more diffused as compared to the electrons of the o-bonds.The reagents which want to react with alkenes attack the π electrons and break it down.The electrons of o-bond are on the joining line of two nuclei and are held tightly.

Raney nickel is prepared by treating an alloy of Ni and AI with caustic soda.Ni is obtained in the powder form and has high surface activity.This is called as Raney pickel.It functions better at 100ᵒC and pressure of 3 atm.

In the laboratory we use this reaction as one of the synthetic methods to convert unsaturated hydrocarbons into saturated ones.It is a quantitative reaction and helps us to measure be amount of unsaturation in the alkenes.On industrial scale vegetable oils are converted into vegetable ghee.

When an unsymmetrical molecule is added to unsymmetrical alkene the positive part of adding molecule goes  to that carbon atom which has already greater number of hydrogen atoms.
CH₃-CH=CH₂+HBr…………………….CH₃-CH-CH₃
Br

Alkenes add one water molecule in the presence of dil.H₂SO₄ at 100ᵒC to give alcohols.The reaction takes place according to Markownikoff’s rule.

When vicinal dihalides are reacted with alc.KOH two molecules of halogen acid are eliminated and triple bond is produced.
CH₂-CH₂+KOH……..alcoholic……… CH=CH+2HBr
Br     Br 

When we do the electrolysis of potassium salts of maleic acid acetylene is collected at the anode while KOH at the cathode.

Ethyne burns in oxygen to give oxyacetylene flame which is used for welding purposes and maintains the temperature of 3500ᵒC.

Two molecules of acetylene give viny acetylene in the presence in the presence of Cu₂CI₂ and NH₄CI.Vinyl acetylene adds HCI to give chloroprene which on polymerization gives synthetic rubber.

Sodamide substitutes the acidic hydrogens by sodium metal to give sodium salts of alkynes.They are used to get higher alkynes by reacting with alkyl halides.

Ammonical solution of AgNO₃ reacts only with those alkynes which have acidic hydrogen in them.1-butyne will give the white precipitate of silver salt but 2-butyne will not.
CH₃-C=CH-CH₃+AgNO₃+NH₄OH……………..No reaction

1-Butyne has acidic hydrogen which gives red ppt.with this reagent but 2-butyne does not give the reaction.

Acetylene is used for welding purposes preparing many organic compounds manufacture of synthetic polymer and for the artificial ripening of fruits.

Ammonical solution of Cu₂CI₂ and of AgNO₃ give red and white precipitates  respectively  with ethyne but not with ethene.