Hydrocarbons Question Answers: NCERT Class 11 Chemistry

Chlorination of methane proceeds via a free radical chain mechanism. The whole reaction takes place in the given three steps.

Step 1: Initiation:

The reaction begins with the homolytic cleavage of Cl- Cl bond as:

Step 2: Propagation:

In the second step, chlorine free radicals attack methane molecules and break down the C- H bond to generate methyl radicals as:

These methyl radicals react with other chlorine free radicals to form methyl chloride along with the liberation of a chlorine free radical.

Hence, methyl free radicals and chlorine free radicals set up a chain reaction. While HCl and CH₃Cl are the major products formed, other higher halogenated compounds are also formed as:

Step 3: Termination:

Formation of ethane is a result of the termination of chain reactions taking place as a result of the consumption of reactants as:

Hence, by this process, ethane is obtained as a by-product of chlorination of methane.

The following structural isomers are possible for C₄H₈ with one double bond:

The IUPAC name of Compound

(I) is But-1-ene, Compound

(II) is But-2-ene, and Compound

(III) is 2-Methylprop-1-ene.

(b) The following structural isomers are possible for C₅C₈ with one triple bond are as follows:

The IUPAC name of Compound

(I) is Pent-1-yne, Compound

(II) is Pent-2-yne, and Compound

(III) is 3-Methylbut-1-yne.

(i) Pent-2-ene undergoes ozonolysis as:

The IUPAC name of Product (I)is butan-2-one and Product (II)is Pentan-2-one.

The IUPAC name of Product (I) is benzaldehyde and Product (II) is propanal.

During ozonolysis, an ozonide having a cyclic structure is formed as an intermediate which undergoes cleavage to give the final products. Ethanal and pentan-3-one are obtained from the intermediate ozonide. Hence, the expected structure of the ozonide is:

This ozonide is formed as an addition of ozone to 'A'. The desired structure of 'A' can be obtained by the removal of ozone from the ozonide. Hence, the structural formula of 'A' is:

The IUPAC name of 'A' is 3-Ethylpent-2-ene.

It is given that, 'A' on ozonolysis gives two moles of an aldehyde of molar mass 44 u. The formation of two moles of an aldehyde indicates the presence of identical structural units on both sides of the double bond containing carbon atoms. Hence, the structure of 'A' can be represented as:

XC = CX

There are eight C-H σ bonds. Hence, there are 8 hydrogen atoms in 'A'. Also, there are three C-C bonds. Hence, there are four carbon atoms present in the structure of 'A'.

Combining the inferences, the structure of 'A' can be represented as:

'A' has 3 C-C bonds, 8 C-H σ bonds, and one C-C π bond.

Hence, the IUPAC name of 'A' is But-2-ene.

Ozonolysis of 'A' takes place as:

The final product is ethanal with molecular mass

= [(2x12) + (4x1) + (1x16)]

= 44u

As per the given information, propanal and pentan-3-one are the ozonolysis products of an alkene. Let the given alkene be 'A'. Writing the reverse of the ozonolysis reaction, we get:

The products are obtained on the cleavage of ozonide 'X'. Hence, 'X' contains both products in the cyclic form. The possible structure of ozonide can be represented as:

Now, 'X' is an addition product of alkene 'A' with ozone. Therefore, the possible structure of alkene 'A' is:

Combustion can be defined as a reaction of a compound with oxygen.

Hex-2-ene is represented as:

The dipole moment of cis-compound is a sum of the dipole moments of C-CH₃ and C-CH₂CH₂CH₃ bonds acting in the same direction. The dipole moment of trans-compound is the resultant of the dipole moments of C-CH₃ and C-CH₂CH₂CH₃ bonds acting in opposite directions. Hence, cis-isomer is more polar than trans-isomer. The higher the polarity, the greater is the intermolecular dipole-dipole interaction and the higher will be the boiling point. Hence, cis-isomer will have a higher boiling point than trans-isomer.

Benzene is a hybrid of resonating structures given as:

Benzene is a hybrid of resonating structures. All six carbon atoms in benzene are sp² hybridized. The two sp² hybrid orbitals of each carbon atom overlap with the sp² hybrid orbitals of adjacent carbon atoms to form six sigma bonds in hexagonal plane. Left sp² hybrid orbital on each carbon atom overlaps with the s-orbital of hydrogen to form six sigma C-H bonds. Left unhybridized p-orbital of carbon atoms has the possibility of forming three π bonds by the lateral overlap of  C₁ - C₂,  C₃- C₄,  C₅ - C₆, or  C₂ - C₃,  C₄-C₅, C₆-C_1.

The six π's are delocalized and can move freely about the six carbon nuclei. Even after the presence of three double bonds, these delocalized π-electrons stabilize benzene.

A compound is said to be aromatic if it satisfies the following three conditions:

(i) It should be cyclic so that each p orbital should overlap with adjacent p orbital to become aromatic.

Cyclic Compound

(ii) It should be planar i.e., all adjacent p orbitals must be aligned so that the p electron density can be delocalized.

Cyclooctatetraene

(iii) It should be completely conjugated i.e, Aromatic compounds must have a p orbital on every atom and each must overlap with adjacent p orbitals.

Conjugated ring

(iv) The total number of π–electrons present in the ring should be equal to (4n + 2), where n = 0, 1, 2 … etc. This is known as Huckel’s rule.

(i) For the given compound, the number of π-electrons is six. But only four π-electrons are present within the ring. Also there is no conjugation of π-electrons within the ring and the compound is not planar in shape. Hence, the given compound is not aromatic in nature.

 (ii) For the given compound, the number of π-electrons is four. By Huckel's rule,

4n + 2 = 4

4n = 2

n = ½

For a compound to be aromatic, the value of n must be an integer (n = 0, 1, 2…), which is not true for given compound. Hence, it is non aromatic in nature.

(iii) For the given compound, the number of π-electrons is eight. By Huckel's rule,

4n + 2 = 8

4n = 6

n = 3/2

For a compound to be aromatic, the value of n must be an integer (n = 0, 1, 2…). Since the value of n is not an integer, the given compound is not aromatic in nature.

(i) Benzene can be converted into p-nitrobromobenzene as:

(ii) Benzene can be converted into m-nitrochlorobenzene as:

(iii) Benzene can be converted into p-nitrotoulene as:

(iv) Benzene can be converted into acetophenone as:

1° carbon atoms are bonded to only one carbon atom, i.e., they have only one carbon atom as their neighbour. The given structure has five 1° carbon atoms and fifteen hydrogen atoms are attached to it.

2° carbon atoms are bonded to two carbon atoms, i.e., they have two carbon atoms as their neighbours. The given structure has two 2° carbon atoms and four hydrogen atoms are attached to it.

3° carbon atoms are bonded to three carbon atoms, i.e., they have three carbon atoms as their neighbours. The given structure has one 3° carbon atom and also one hydrogen atom is attached to it.

Alkanes experience inter-molecular Van der Waals forces. The stronger the force, the greater will be the boiling point of the alkane.

As branches increases, the surface area of molecule decreases resulting in a small area of contact. As a result, the Van der Waals force also decreases which can overcome at a relatively lower temperature. Hence, the boiling point of an alkane chain decreases with an increase in branches.

Addition of HBr to propene is an example of an electrophilic substitution reaction.

Hydrogen bromide provides an electrophile H+. This electrophile attacks the double bond to form 1° and 2° carbocations.

Secondary carbocations are more stable. Hence, the former predominates since it will form at faster rate. Thus, in the next step, Br^- attacks the carbocation to form 2 - bromopropane as major product.

This reaction follows Markovnikov's rule where the negative part of the addendum is attached to the carbon atom having a lesser number of hydrogen atoms.

In the presence of benzoyl peroxide, an addition reaction takes place anti to Markovnikov's rule. The reaction follows a free radical chain mechanism as:

Secondary free radicals are more stable than primary radicals. Hence, the former predominates since it forms at a faster rate. Thus, 1 - bromopropane is obtained as the major product.

In the presence of peroxide, Br free radical acts as an electrophile. Hence, two different products are obtained on addition of HBr to propene in the absence and presence of peroxide.

o-xylene has two resonance structures:

All three products, i.e., methyl glyoxal, 1, 2-dimethylglyoxal, and glyoxal are obtained from two Kekule structures. Since all three products cannot be obtained from any one of the structures, this proves that o-xylene is a resonance hybrid of two Kekule structures (I and II).

Acidic character of species is defined on the basis of ease with which it can lose its H-atoms.

The hybridization state of carbon in the given compound is:

As the s-character increases, the electronegativity of carbon atom increases and the electrons of C-H bond pair lie closer to the carbon atom. As a result, partial positive charge of H^- atom increases and H⁺ ions are set free.

The s-character increases in the order:

sp3 < sp2 < sp

Thus, the decreasing order of acidic behaviour is Ethyne > Benzene > Hexane.

Benzene is a planar molecule having delocalized electrons above and below the plane of ring. Hence, it is electron-rich. As a result, it is highly attractive to electron deficient species i.e., electrophiles.

Therefore, it undergoes electrophilic substitution reactions easily. Nucleophiles are electron-rich. Hence, they are repelled by benzene. Hence, benzene undergoes nucleophilic substitutions with difficulty.

The basic skeleton of 2-methylbutane is shown below:

On the basis of this structure, various alkenes that will give 2-methylbutane on hydrogenation are:

Electrophiles are reagents that participate in a reaction by accepting an electron pair in order to bond to nucleophiles.

The higher the electron density on a benzene ring, the more reactive is the compound towards an electrophile, E⁺ (Electrophilic reaction).

(a) The presence of an electron withdrawing group (i.e., NO_2- and Cl_-) deactivates the aromatic ring by decreasing the electron density.

Since NO_2- group is more electron withdrawing (due to resonance effect) than the Cl- group (due to inductive effect), the decreasing order of reactivity is as follows:

Chlorobenzene > p - nitrochlorobenzene > 2, 4 - dinitrochlorobenzene

(b) While CH_3- is an electron donating group, NO_2- group is electron withdrawing. Hence, toluene will have the maximum electron density and is most easily attacked by E⁺.

NO_2- is an electron withdrawing group. Hence, when the number of NO_2- substituents is greater, the order is as follows:

Toluene > p-CH₃-C₆H₄-NO₂, p -O₂ N-C₆H₄-NO₂

The ease of nitration depends on the presence of electron density on the compound to form nitrates. Nitration reactions are examples of electrophilic substitution reactions where an electron-rich species is attacked by a nitronium ion (NO_2-).

Now, CH_3- group is electron donating and NO_2- is electron withdrawing. Therefore, toluene will have the maximum electron density among the three compounds followed by benzene. On the other hand, m-Dinitrobenzene will have the least electron density. Hence, it will undergo nitration with difficulty. Hence, the increasing order of nitration is as follows:

The addition of an ethyl group is done in the ethylation reaction of the benzene ring. Such a reaction is called a Friedel-Craft alkylation reaction. This reaction takes place in the presence of a Lewis acid.

Any Lewis acid like anhydrous FeCl₃, SnCl₄, BF₃ etc. can be used during the ethylation of benzene.

Wurtz reaction is limited for the synthesis of symmetrical alkanes (i.e., alkanes with an even number of carbon atoms). In this reaction, two similar alkyl halides are taken as reactants and an alkane, containing double the number of carbon atoms, are formed.

Example: Wurtz reaction cannot be used for the preparation of unsymmetrical alkanes because if two dissimilar alkyl halides are considered as the reactants, then a mixture of alkanes is obtained as the products. Since the reaction involves free radical species, a side reaction also occurs to produce an alkene. For example, the reaction of bromomethane and iodoethane gives a mixture of alkanes.

The boiling points of alkanes (obtained in the mixture) are very close. Hence, it becomes difficult to separate them.