KPK Board 12th class Chemistry Ch 16 Hydrocarbons questions answers

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What is the difference between aliphatic and aromatic hydrocarbons?
Aliphatic Hydrocarbons: organic compound having open chain or cyclic structure. They may be straight, branched or non aromatic rings. They can be joined by single bond (alkane) double bond (alkene) or triple bond (alkyne).They may have localized or delocalized multiple bonds. Aliphatic hydrocarbons can give electrophilic substitutions additions reactions easily.

Aromatic Hydrocarbons: The compound having at least one benzene ring in their structure. They always have a benzene ring in their structure. They are also having alternate double bound in the cyclic structure. They always have delocalized bonding system. Aromatic hydrocarbons major reaction is electrophilic aromatic substitution.

What is meant by resonance?
The representation of real structure as the weighted average of two or more contributing structure is called resonance.


Delocalization of π electrons is called resonance.

How do you detect the unsaturation in Alkenes?
When an alkenes is treated with bromine water in presence of in inert solvent yield dihalides and decolourised the reddish colour of bromine water which is a test used indicated instauration in alkene.
Why benzene is less reactive than ethane but more than reactive than ethane?
Benzene is less reactive than alkenes although both have double bound .The double bound of alkenes is more exposed to than attack by an electrophile  than benzene because the double bound of aikene is localized over two carbon atoms while in benzene the pi could is formed which is delocalized on the wholi ring which make the benzene ring more stable but less reactive than alkenes.
Why halogens are deactivating group although they are artho-para directing?
Halogens are most electronegative elements so when they are attached with benzene ring they attract the ring electrons towards itself which results decrease the electron density on benzene ring deactivating for further substitution but at the same time halogen atoms also have lone pair of electrons which can be delocalize over benzene ring resulting increase electron density at artho-para position. So due to greater electronegative halogens are deactivating halogens but due to presence of lone pair electrons they are artho-para directors.

Why Ethyne is less reactive towards elecrtrophilic addition reaction than ethene?
Ethyne is less reactive towards elecrtrophilic addition reaction than ethane this less reactivity of ethyne is due to the following two reasons.

  1. In ethene the double bond consist of one pi π and one sigma bond. The pi electrons could is above and below the plane of atoms and is thus exposed two electrophilic attacks.
  2. The carbon atoms in ethyne are sp-hybridized while is ethene are sp2-hybridized.
What is meant by chirality?
The term that has been recently used to described such molecules having no plane of symmetry. Thus symmetrical molecules are called chiral molecules and this property of molecule is called chirality.
Why there are no geometrical isomers of but-1-ene?
The structure of one butane is;


One of the basic conditions for geometric isomerism is that “there must be two different atoms attached with each double bonded carbon atom” whereas in case of one butene linked two hydrogen atoms attached with one of the doubly bonded carbon atom thus it does not have any geometrical isomer.

Why cyclopropane is more reactive than propane?
If we look at the structure of both cyclo-propane and propane it is clear that in the cyco-propane the overlap of orbital is not maximum thus result in high angle strain witch mean lesser stabilities, more reactivity then propane in which there is no angle strain. i.e. why cyslo-propane is more reactive as compare to normal propane.
Differentiate between Homolytic and Heterolytic fission?
Homolytic fission: the fission in which covalently bonded atoms get one electron to become a radical. It results in the formation of radicals it occurs between similarity boned atoms. Electronegativity difference between atoms is negligible.

Heterolyatic Fission: the fission in which the covalently bonded electrons completely sifts towards one atom. It results in the formation of ions. It occurs only in different atoms. electrons pairs move toward more electronegative atom.

Write doen the resonance structure of benzene?
Write down twoe reasons of meta-directing effect?
More electronic density at Meta position. The phenonium ion is more stable when attack at meta position.
Who can you distinguish one-alkyne from other non-terminal alkynes?
The following test is used to distinguish between the two alkyne. When amonical solution of corpus chloride and silver nitrate are treated with one link alkyne is forms Ag-Acetyl ides and Cu-Acetyl ides.

CH3-CH2-CCH + NH4OH + Cu2Cl2 àCH3-CH2-CC-Cu

H-CC-CC-H + 2AgNO3 + NH4OH à Ag-CC-CC-Ag

Not terminal alkyne does not form acetyliades. Thus this reaction can be used to distinguish between them to.

Why Lindler’s catalyst is used in hydrogenation of alkynes?
Lindler’s catalyst (Ni/Pt//Pd-C/BaSO4or quinoline) is used for partial reduction of alkynes i.e.  Conservation of alkynes to alkenes.CH CH+H2àCH2 =CH

If we do not use this catalyst then hydrogenation of alkynes will always yields alkenes .So, in order to stop the reaction at alkenes stage we use Lindler’s catalyst.

What is the main difference between tautomerism and metameriam?
Both are types of structure isomericsm but basic difference is the tautomers the two isomeric structure always exist in rapid equilibrium and are easily interconvert able due which it is also known as keto-enol tautomerism.


Whereas in  mesmerism there is no such equilibrium exit among two isomers.

CH3-CH2-C=O-CH2-CH3              CH3-C=O-CH2-CH2-CH3

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