Double Bonds, Configurations And Conformations

Question 1

What are the two types of molecular bonding orbital

Answer 1

Pi
Sigma

Question 2

What is a sigma bond

Answer 2

Covalent bond formed from sharing a pair of electrons in a sigma orbital

Question 3

What is a pi bond

Answer 3

Covalent bond formed from sharing a pair of electrons in a pi orbital

Question 4

Draw the distribution of electrons for sp2 hybridised carbon atom

Answer 4

Trigonal planar shape
2p2 hybridised (since it wants to form 3 bonds)
The 2pz orbital doesn’t combine (its unhybridised)

Question 5

What does hybridisation of orbitals do

Answer 5

It takes all of the electron density of the carbon atom and divides it up equally into hybridised molecular orbitals

Question 6

Draw the energy level diagram for the molecular orbitals in ethene

Answer 6

Question 7

Draw a diagram of the pi and sigma bond in ethene

Answer 7

Question 8

What is bond length determined by

Answer 8

The more electrons there are in a bond= two nuclei can become closer = shorter bond length

Question 9

Why can’t rotation occur around a c=c bond

Answer 9

This is because the pi bond isn’t cylindrically symmetrical

Question 10

How do cis/trans isomers form (aka geometric isomerism)

Answer 10

Change in configuration
The groups attached to one carbon can swap places- to do this the pi bond would have to break

Cis= same groups are on the same side
Trans= same groups are on opposite sides

Question 11

What is a node

Answer 11

A point where the probability of finding an electron is zero

Question 12

What arrangement of nodes can there be for benzene

Answer 12

Question 13

Show the electrons in the bonding and anti bonding orbitals of benzene

Answer 13

Question 14

What are heterocyclic aromatic systems

Answer 14

They contain nitrogen or oxygen instead of carbon in a ring

Question 15

Show the distribution of electrons for pyrimidine

Answer 15

Question 16

Show the distribution of electrons in pyrrole

Answer 16

Nitrogen will contribute 2 electrons so there 6 electrons in the bonding orbital

Question 17

How would atoms in a molecule share a greater number of electrons

Answer 17

Question 18

What are single, double and triple bonds made up of in terms of pi and sigma bonds

Answer 18

Question 19

What are conjugated bonds

Answer 19

Question 20

How are the electrons in a molecule of benzene arranged

Answer 20

Each carbon has 3 sigma bonds and one valence electron (which isn’t directly involved in bonding) which occupies a p atomic orbital
The p orbitals on each of the 6 carbon atoms in the ring overlap to form a single uninterrupted pi orbital (which form 2 ‘donuts’ above and below the plane of the carbon ring- these e- are free to move anywhere that forms the lowest energy in the pi orbital

Question 21

What is a delocalised system

Answer 21

A delocalized system in chemistry refers to a situation where electrons are not confined to a single atom or a single covalent bond. Instead, these electrons are spread out or “delocalized” over several atoms

Question 22

How does a conjugated system arise

Answer 22

Question 23

What do atomic orbitals represent

Answer 23

Atomic orbitals represent electron wave functions
Orbital overlap involves combination of these wave functions

Question 24

How does the bonding in hydrogen work

Answer 24

H atom has one electron in 1s orbital
1s orbitals of two H atoms overlap to form sigma orbital
Both electrons from each hydrogen atom reside here

Question 25

Why does a bonding and anti bonding orbital form

Answer 25

The number of orbitals have to stay the same
So since 2 orbitals are overlapping to form a molecular orbital, 2 hybridised orbitals would form -> a bonding orbital and anti bonding orbital

Question 26

How can electrons jump from the bonding orbital to the anti bonding orbital

Answer 26

Electrons can jump from a bonding orbital to an anti-bonding orbital through the absorption of energy. This process typically occurs when the molecule absorbs a photon of light (electromagnetic radiation) with energy equal to the energy gap between the bonding and anti-bonding orbitals.

Question 27

How do molecular orbitals allow an atom to form bonds with other atoms to form a molecule

Answer 27

When atomic orbitals are hybridised to form hybridised molecular orbitals they would be on the same energy level
Each orbital would have one electron residing in it
This leaves room for another electron to be provided by another atom to form a bond
The number of hybridised orbitals formed would depend on the number of bonds that need to be formed
E.g methane would have 4 hybridised orbitals and would undergo sp3 hybridisation to form 4 bonds with 4 hydrogen atoms

Question 28

Why is there a 2p orbital left unhybridised in carbon when it forms ethene

Answer 28

Sp2 hybridisation-> occurs to form double bonds
The orbital left unhybridised will be used to form a pi bond
Also because to form ethene, only 3 molecular orbitals are needed to form 3 bonds with 3 other atoms

Question 29

How are orbital diagrams for a molecule drawn

Answer 29

Question 30

What’s the difference between a sigma orbital and a hybridised molecular orbital

Answer 30

A sigma orbital is a molecular orbital formed by the head-on overlap of atomic orbitals along the internuclear axis, while a hybridized molecular orbital is formed by the combination of atomic orbitals on the same atom to create new orbitals that are used to form sigma bonds and accommodate lone pairs in a molecule