Hydrocarbons: Short questions and answers (Part 1/2)

Question 1: Why carbon is sp³ hybridized in most of organic compound?
Answer:

The term 'sp³' refers to intermixing of one 's'  and three 'p' atomic orbitals of an atom to produce four new hybrid orbitals.  In most of organic compounds, carbon is bonded with four other  atoms through single covalent bonds. 

This cannot be achieved by carbon having two electrons in 2p atomic orbital. In order to make four bonds, one electron from '2s' is excited to the '2p: atomic orbital. Thus, carbon becomes tetravalent. However, these four orbitals make similar bonds (equivalent). In order to justify equivalency, it is assumed that carbon undergoes sp³ hybridization, in which one s and three p atomic orbitals intermix with other to produce four sp³ hybridized orbitals which have same characteristics.

Question 2: How is pi bond formed in alkene and alkyne?
Answer.
Pi bond in Alkenes:

In alkene, carbon atoms bonded twice with each other are sp² hybridized. Each sp² hybridized carbon has three sp² hybridized and one un-hybridized orbitals. Since, sp² hybridized orbitals are formed due to mixing of one '2s' and two '2p' atomic orbitals leaving one 2p atomic orbital hybridized.  Three sp² hybridized orbitals of the carbon atom explain formation of three sigma bonds with other atoms. Remaining 2p atomic orbitals of  adjacent sp2 hybridized carbon atoms overlap sideways to form a pi bond.

A pi-bond is formed after the formation of sigma bond. It is a weaker bond due to partial overlap. It restricts the rotation around carbon - carbon single bond. It is never formed from hybrid orbitals.

Pi bond in Alkyne

In alkyne, carbon atoms bonded thrice with each other are sp hybridized. Each sp hybridized carbon has two sp hybridized and two un-hybridized orbitals. Since, sp hybridized orbitals are formed due to mixing of one '2s' and one '2p' atomic orbitals leaving two 2p atomic orbitals hybridized. 

Two sp hybridized orbitals of the carbon atom explain formation of two sigma bonds with other atoms. Remaining two '2p' ' atomic orbitals of  sp hybridized carbon atom undergo parallel overlapping with corresponding two' 2p' orbitals of adjacen sp  hybridized carbon atom to form two pi bonds.

Question 3.

What is cis-trans isomerism? (out of smart syllabus)
Answer. 

Isomerism that arises due to restricted rotation somewhere in a molecule of a compound is called cis- trans isomerism or geometric isomerism.
Compound which have restricted rotation about carbon-carbon bond are of two types.
1. Compounds containing carbon - carbon double bond
2. Compounds containing carbon - carbon single bond in cyclic ring.

Condition for geometric isomerism
For geometric isomerism, two groups attached to same carbon of double bond or any two carbon of cyclic ring must be different.

Question 4: Why are alkanes relatively chemical inert?
Answer.

Alkanes are weakly reactive with other compounds due to two main factors.
i. Inertness of sigma bonds
ii. Non polar bond
i. Inertness of sigma bonds
In alkanes, carbon atom is sp³ hybridized and each carbon makes four sigma bonds with other atoms. These four sigma bonds are more stable than any other kind of bond because of greater overlapping of atomic orbitals. These bonds are quite strong and require a large amount of energy to break. Hence, alkanes are less reactive.
ii. Non polar bond
In alkanes, there are carbon - carbon single bonds and carbon - hydrogen single bonds. As electronegativity difference between carbon and hydrogen is very small. Therefore, these bonds do not carry significant amount of charge and cannot attract other molecule or ions. Hence, Alkane are less reactive.

Question 5:Alkenes undergo addition reactions while alkane don't. Why?
Answer.
Alkenes are unsaturated while alkanes are saturated hydrocarbons. Alkenes are reactive and undergo addition reaction while alkanes do not undergo. This is because, alkenes contain pi bonds which can easily be broken. On the other hand, alkanes contain only sigma bonds which are not easy to break. Also, electron density of pi bond lie above and below the plane of bonding atoms. This increases the availability of electrons for electrophilic addition reaction and hence alkenes undergo addition reactions.

Question 6: What is stereoisomerism?  (out of smart syllabus)
Answer.
The molecules which have same molecular formulas, same connectivity of atoms, but different arrangements of atoms  in three dimensional space are called stereoisomers and this phenomenon is called stereoisomerism.
Stereoisomerism is of two types.
Geometric isomerism.
Optical isomerism.

Question 7. What is optical isomerism ? How optical isomerism arises?

Answer.
Optical isomerism 
The molecules which have same molecular formulas, same connectivity of atoms, and different arrangements of atoms in three dimensional space as well as non-superimposable mirror images are called optical isomers and this phenomenon is called optical isomerism.
How optical isomerism arises? 
Optical isomerism arises due to presence of chiral center. The chiral center is carbon atom attached with four different groups. Due to four groups, there is no symmetry anywhere in the molecule. 

Optical isomers have mostly same physical properties and often distinguished due to difference in optical activity in which one isomer rotate plane polarized light in clockwise while other isomer rotate plane polarized light in anti clockwise direction.
If two groups attached to central carbon become identical, there will be a plane of symmetry (no more chiral center or chiral carbon) and optical isomerism vanished away.

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