CHEMISTRY Class XI Chapter : Hydrocarbons Free Notes
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| CHEMISTRY Class XI Chapter : Hydrocarbons Free Notes |
Classify the hydrocarbons according to the carbon – carbon bond
Hydrocarbons are categorized into three categories according to the carbon – carbon bond that exists between then- (a) saturated hydrocarbon (b) Unsaturated hydrocarbon (c) Aromatic hydrocarbon.
What are cycloalkanes?
When carbon atoms form a closed chain or a ring, they are termed as cycloalkanes.
The boiling point of hydrocarbons decreases with increase in branching. Give reason.
Unsaturated hydrocarbon compounds contain carbon – carbon double or triple bonds. The Ï€-bond is multiple bond is unstable and therefore addition takes place across the multiple bonds.
What is hydrogenation?
Dihydrogen gas gets added to alkenes and alkenes in the presence of finely divided catalysts like Pt, Pd or Ni to form alkanes. This process is called hydrogenation.
Give the IUPAC name of the lowest molecular weight alkane that contains a quaternary carbon.
2, 2-dimethyl propane.
Methane does not react with chlorine in dark. Why?
Chlorination of methane is a free radical substitution reaction. In dark, chlorine is unable to be converted into free radicals, hence the reaction does not occur.
How will you prepare isobutane?
Isobutane is obtained by decarboxylation of 3-methyl butanoic acid with soda lime at 630K.
N – pentane has higher boiling point than neopentane but the melting point of neopentane is higher than that of n – pentane.
Because of the presence of branches in neo-pentane the surface area and van der walls forces of attraction are very weak in neopentane than in n-pentane. Therefore the b.p of neopentane is lower than that of n-pentane. M.P depends upon the packing of the molecules in the crystal lattice. Since neopentane are more symmetrical than n-pentane therefore, it packs much more closely in the crystal lattice than n-pentane and hence neopentane has much higher m.p than n-pentane.
The boiling point of alkanes shows a steady increase with increase in molecular mass. Why?
This is due to the fact that the intermolecular van der walls forces increase with increase of the molecular size or the surface area of the molecule.
Pentane has three isomers i.e; pentane, 2-methyl butane and 2,2-dimethyl propane . The b.p of pentane is 309.1K whereas 2,2-dimethyl propane shows a b.p of 282.5k. Why?
With the increase in number of branched chains, the molecule attains the shape of a sphere. This results in smaller area of contact and therefore weak inter molecular forest between spherical molecules, which are overcome a relatively lower temperatures.
Which conformation of ethane is more stable?
Staggered conformation.
All the four C-H bonds in methane are identical. Give reasons.
The four C-H bonds of methane are identical because all of these are formed by the overlapping of the same type of orbital’s i.e; hybrid orbital’s of carbon and sorbital of hydrogen.
Why is alkene considered a weaken molecule than alkane?
The presence of weaker π-bond makes alkenes unstable molecules in comparison to alkane and thus alkenes can be changed into single bond compounds by combining with the electrophoilic reagents.
What is the origin of geometrical isomerism in alkenes?
Geometrical isomerism in alkenes is due to the lack of free rotation of the doublebonded carbon atoms due to the double bond between them.
What are geometrical isomers?
The restricted rotation of atoms or groups around the doubly bonded carbon atoms gives rise to different geometries of such compounds. The stereo isomers of this type are called geometrical isomers.
What is a Lindlars’ catalyst?
Partially deactivated palletized charcoal is known as Lindlar’s catalyst.
Stale Markownikov’s Rule.
It states that when a polar compound is added to an unsymmetrical alkenes, or alkynes positive part goes to the most substituted carbon atom and negative part goes to the least substituted carbon atom.
Ethyne is acidic in nature in comparison to ethene and ethane. Why is it so?
Hydrogen atoms in ethyne are attached to the sp hybirdised carbon atoms whereas they are attached to sp2 hybridized carbon atoms in ethene and sp3 hydridised carbons in ethane. Due to the maximum percentage of s – character (50%), the sp hybridized orbital’s of carbon atoms in ethyne molecules have highest etcetronegativity : Which attracts the shared pair of the C-H bond of ethyne to a greater extent than that of the sp2 hybridized orbital’s of carbon in ethene and the sp3 hybridized orbital of carbon in ethane. Thus in ethyne molecule, hydrogen atoms can be liberated as protons more easily as compared to ethene and ethane.
How would you distinguish between butyne – 1 and butyne – 2?
Butyne – 1 (CH3CH2C ≡ CH), having an acetylene hydrogen atom will give white precipitate with ammonical silver nitrate and red precipitate with ammonical cuprous chloride. On the other hand, butyne – 2 (CH3C ≡ C CH3) having no acetylene hydrogen atom does not respond to either of the two reagent.
What are benzenoids?
Aromatic hydrocarbon compound containing benzene ring are known as benzenoids.
What do you mean by delocalization?
Delocalisation – Delocalisation implies that pairs of bonding electrons extend over three or more atoms and belong to the whole molecule. Delocalized Ï€-orbitals are much larger than the localized Ï€-orbitals and are therefore more stable.
What do you understated by Resonance energy?
The difference between the energy of the most stable contributing structure and the energy of the resonance hybrid is known as resonance energy. In case of benzene, the resonance hybrid has (147KJ/mol-1) less energy than either A to B. Thus resonance energy of benzene is 147KJ/mole.
Although benzene is highly unsaturated; it does not undergo addition reactions. Give reason.
Unlike olefins, π-electrons of benzene are delocalized (resonance) and hence these are uncreative towards addition reactions.
How is aromaticity of a compound judged?
The following characteristics decides aromaticity of a compound-:
(i) Planarity
(ii) Complete delocalization of the π-electrons in the ring.
(iii) Presence of (4n+2) π electrons in the ring where n is an integer (n=0, 1, 2 ----)
How will you account for the structure of benzene?
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| How will you account for the structure of benzene? |
What are electrophilic substitution reactions?
Those reactions in which weaker electrophile are replaced by a stronger electrophile are called electrophilic substitution reactions.
How will you distinguish between Ethene and benzene
Ethene discharges bromine water colour and Baeyer’s reagent colour while benzene does not.
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