Molecular Orbitals And Hybridisation And Nucleophilic Substitution Reactions

Question 1

When do molecular orbitals form?

Answer 1

When two atomic orbitals combine. They encompass the entire molecule. The number of molecular orbitals formed is always equal to the number of atomic orbitals that combine.

Question 2

Sigma bonds

Answer 2

Form when atomic orbitals combine along the x-axis of the bond by the end-on overlap.

Question 3

Pi bonds

Answer 3

Form when atomic orbitals lying perpendicular to the axis of the bond combine by a side-on overlap to form a molecular orbital. They form when multiple bonds form in a molecule, for example in a double bond.

Question 4

What are the types of bonds in a double bond

Answer 4

A pi and sigma bond

Question 5

Which is stronger pi or sigma bonds?

Answer 5

The side-on overlap is less efficient than an end-on overlap and so pi bonds are weaker than sigma bonds.

Question 6

What kind of hybridisation do alkanes have?

Answer 6

Sp3 and have sigma bonds as the 1s orbitals overlap end on.

Question 7

What kind of hybridisation is in alkenes?

Answer 7

Sp2 and one unhybridised 2p orbital. They have sigma bonds between the hybrid orbitals. They unhybridised p orbitals on each of the carbon atoms overlap side on to form a pi bond.

Question 8

What kind of hybridisation is in alkynes?

Answer 8

One sp hybrid and two unhybridised p orbitals.

Question 9

What kind of hybridisation is in benzene?

Answer 9

Each carbon is sp2 hybridised and these hybrid orbitals form sigma bonds with neighbouring carbon atoms. The remaining unhybridised p orbitals on each of the six carbon atoms overlap side on to create a pi molecular orbital, often called a pi electron cloud. This cloud has a lobe above and below the plane of the carbon ring and contains one electron from each carbon atom. These electrons are delocalised.

Question 10

3 types of chemical bonds that exist in compounds

Answer 10

• pure covalent (electrons shared equally)
• polar covalent
• ionic (electrons transferred from one atom to another)

Question 11

Bonding molecular orbital in a non-polar covalent bond.

Answer 11

Molecular orbital is symmetrical around nuclei

Question 12

Bonding molecular orbital in a polar covalent bond

Answer 12

Molecular orbital is asymmetrical but still encompasses both nuclei

Question 13

Bonding molecular orbital in an ionic bond.

Answer 13

Molecular orbital is only around the thing that was the most electronegative element.

Question 14

Substitution reactions

Answer 14

One where one atom is substituted for another

Question 15

What makes the carbon atom attached to the halogen susceptible to attack from nucleophiles?

Answer 15

The bond polarity

Question 16

What do monohaloalkanes take part in nucleophilic substitution reactions with?

Answer 16

• alkalis to form alcohols
• alcoholic alkoxides to form ethers
• ethanolic cyanide to form nitriles

Question 17

Reaction mechanisms

Answer 17

The mechanism for a reaction shows the steps which take place and the movement of electrons involved in making/breaking bonds.

Question 18

First order nucleophilic substitution reaction

Answer 18

There is only one species in the RDS. The reaction occurs in a minimum of two steps via a Trigonal planar carbocation intermediate.

Question 19

Second order nucleophilic substitution reaction

Answer 19

There are two species involved in the RDS. The reaction proceeds via single five centred Trigonal bipyramidal transition stage.

Question 20

Steric hindrance

Answer 20

The hindrance of a reaction due to a molecules shape/orientation in space, making it difficult for the partially positive carbon atom to be attacked by a nucleophile. Look for bulky alkyl groups.

Question 21

Inductive stabilisation of an intermediate carbocation

Answer 21

Alkyl groups have an inductive effect where they push the electron in a covalent bond away from them. This can stabilise a positive charge on a neighbouring carbon atom. The more alkyl groups attached to a positively charged carbon atom, the bigger the stabilisation effect.

Question 22

Why is the SN2 mechanism often dominant in primary monohaloalkanes?

Answer 22

• the carbon with the halogen attached is not sterically hindered
• the carbocation which would be formed in a SN1 mechanism would not be very stable as it would only experience the inductive stabilisation effect from one alkyl group. This works against the SN1 mechanism.

Question 23

Why is the SN1 mechanism often dominant in tertiary monohaloalkanes?

Answer 23

• the carbon atom with the halogen atom attached is sterically hindered. This works against the SN2 mechanism.
• the carbocation which forms in the SN1 mechanism is stabilised by the inductive stabilisation effect from three alkyl groups. This favours the SN1 mechanism.

Question 24

How can you determine which mechanism occurs/is more dominant?

Answer 24

It can only be determined by experimental methods