Hybridisation/Hydrocarbons + Cycloalkanes/Haloalkanes

Question 1

What is hybridisation?

Answer 1

hybridisation is the mixing of atomic orbitals to form molecular orbitals
- mixing of 2s with 2p orbitals on a carbon to make 4 hybrid orbitals

Question 2

What are sp3, sp2 and sp hybridised orbitals?

Answer 2

these hybridised orbital occur as the 2s orbital mixes with the (2p) px, py and pz orbitals

sp3
- s orbital mixes with 3 p orbitals
- 25% s character and 75% p character
sp2
- s orbital mixes with 2 p orbitals. there is one unhybridised p orbitals
- 33% s character and 66/67% p character
sp
- s orbital mixes with 1 p orbitals. there is 2 unhybridised p orbitals
- 50% s character and 50% character

Question 3

How are single, double and triple bonds hybridised?

Answer 3

single bond
- sp3 hybridised = have 1 sigma bond
- tetrahedral = bond angle 109.5
double bond
- sp2 hybridised = have 1 sigma bond and 1 pi bond
- trigonal planar = bond angle 120
triple bond 
- sp hybridised = have 1 sigma bond and 2 pi bonds 
- linear = bond angle 180

Question 4

What are sigma and pi bonds?

Answer 4

sigma bond
- formed by hybridised orbitals = caused by the overlap of atomic orbitals

pi bonds
- formed by unhybridised orbitals = caused by unhybridised p orbitals

sigma bonds are stronger than pi bonds

Question 5

What are the properties of single, double and triple bonds?

Answer 5

single

• longest bond
• greatest p orbital
• most dispersed

double
- shorter than single bonds but longer than triple bonds

triple

• shortest bond
• most compact
• spherical orbital = most s character (50%)

Question 6

What are the properties of alkanes, alkenes and alkynes?

Answer 6

alkanes

• single bond = sp3
• least reactive
• no pi bonds = only sigma bonds

alkenes

• double bond = sp2
• more reactive than single bonds but less than triple bonds
• have one pi bond

alkynes

• triple bond = sp
• most reactive
• have two pi bonds

pi bonds increase reactivity

• pi bonds are sideways overlap of p orbitals
• are easy to break = makes the molecule reactive

Question 7

How is rotation around a single, double and triple covalent bond?

Answer 7

single bond
- free rotation = energy barrier is small, no pi bonds

double and triple bonds

• no rotation
• has pi bonds = cannot rotate when pi bonds are present

rotation when a pi bond is present would break the sideways overlap of p orbital = pulls the molecule apart

Question 8

How does straight chain and branched structure affect boiling point of alkanes?

Answer 8

straight chain alkanes have a higher boiling point than branched alkanes

straight chains have more intermolecular interactions
- have more London dispersion forces as atoms are closer together

branched chains have fewer intermolecular interactions
- have fewer London dispersion forces as the atoms are further apart = less contact

Question 9

What are the types of ring strain?

Answer 9

affects the stability of cycloalkanes

angle strain

• strain occurs when the bond angle within a ring is less than 109.5 (less than a tetrahedral bond angle)
• more significant in smaller rings = cyclopropane (3 carbon ring)

torsional strain

• strain is generated when H atoms on adjacent C atoms are eclipsed (exactly opposite)
• more significant/increases with ring size

Question 10

What is eclipsed conformation?

Answer 10

H atom are all lined up

- aligned/exactly on top of each other

Question 11

What is puckering?

Answer 11

puckering is the deformation of the ring
- relieves torsional strain by twisting the C atoms out of the eclipsed conformation = ring bends

pull one C atom up and pushes the opposite C atom down

Question 12

Why is cyclohexane the most stable cycloalkane?

Answer 12

has a high amount of torsional strain but can be deformed (puckered) into a chair conformation
- C-H bonds are made to be staggered

chair conformation - little to no angle or torsional strain
chair conformation - is the lowest energy conformation for cyclohexane (lower than planar)

Question 13

What are the reactions that alkanes can undergo?

Answer 13

combustion
- alkane + oxygen = carbon dioxide + water (+ energy)

halogenation - substitution (1H is substituted for 1 halogen)

• alkane + halogen = haloalkane + hydrogen halide
• haloalkane + halogen = haloalkane + hydrogen halide

dehydration - removal of hydrogen molecule
- alkane = alkene + hydrogen
= requires pressure and high temperature
alkenes can undergo addition reactions to become alkanes (addition of hydrogen molecule)

Question 14

What is heterolytic fission and homolytic fission?

Answer 14

heterolytic fission

• moves two electrons at a time
• double headed arrow

homolytic fission

• moves one electron at a time
• single headed arrow

Question 15

What are the steps in radical chain reaction?

Answer 15

initiation

• generation of radicals
• molecule reacts with itself = each atom takes one electron each

propagation = cyclic

• hydrogen abstraction
• forms hydrogen halide and radical compound
• halogen abstraction
• forms original halide radical and halide compound

reformed radical halide is the chain carrier

termination

• radical concentration is low
• radicals react to form compounds

Question 16

Why are haloalkanes heavier than water?

Answer 16

alkanes are immiscible in water = produce an upper layer (floats)
alkanes are less dense than water
alkanes = London dispersion forces
water = hydrogen bonds

haloalkanes - temporary and permanent dipoles makes them more dense than water = form lower layer