Chapter 13

Question 1

What are alkenes?

Answer 1

unsaturated hydrocarbons that contain one or more carbon double bond
CnH2n
-2 double bonds= diene suffix
-120 degrees
-Trigonal planar
- all atoms are on the same plane

Question 2

What do bonds require?

Answer 2

Orbital overlap
-e.g. Sigma bonds are a head-on overlap of orbitals (single bonds)
S orb - S orb overlap // P orb- P orb overlap // S orb - P orb overlap

Question 3

What is an orbital?

Answer 3

An area with a high likelihood of encountering an electron, holding a maximum of 2 electrons in each orbital

Question 4

What are Pi bonds? and double bonds?

Answer 4

sideways overlap of 2 p orbitals
a double bond is one sigma and one pi bond
- p orbitals sit at 90 degrees to the plane of the molecule to form the pi bond

Question 5

If there is an alcohol group, what bond angle is there?

Answer 5

104.5 because the oxygen has two bonded pairs and two lone pairs, with lone pairs repelling more than bonded pairs
=> 104.5 degrees so it is non linear around the oxygen atom

Question 6

What are stereoisomers

Answer 6

same structural but different arrangement of atoms in space

Question 7

E/Z isomerism

Answer 7

-caused by the C=C double bond (groups attached to each carbon atom are fixed relative to each other and cannot rotate)
- ONLY works if it satisfies both conditions
=> a double bond and different groups attached to each carbon atom of the double bond

Question 8

E isomers and Z isomers

Answer 8

E isomers form when the different groups are diagonal to each other (zigzag skeletal)
Z isomers form when the groups are on the same side as each other (U shaped skeletal= flat double bond)

Question 9

Cis/trans isomerism

Answer 9

a special type of E/Z where each carbon in the double bond MUST be attached to hydrogen
In this case, Cis = Z and Trans = E

Question 10

Cahn-Ingold-Prelog nomenclature

Answer 10

atoms attached to each carbon in the double bond are given priority based upon their atomic number
- If groups of higher priority are on the same side, compound is Z ISOMER
- If groups of higher priority are diagonally placed, compound is E isomer

The higher the atomic number, the greater the priority
- If two atoms attached to a carbon atom are the SAME, then the first point of difference must be looked at

Question 11

Reactivity of alkenes

Answer 11

More reactive than alkanes because of the presence of the pi bond
=> P- electron density is concentrated ABOVE and Below the plane of sigma bond
=> Since it is outside double bond, the pi electrons are more exposed than electrons in sigma bond. Therefore a pi bond readily breaks and alkenes undergo addition reactions relatively easily
=> bond enthalpy of a pi bond is less than a c-c sigma bond

Question 12

Addition of hydrogen to alkenes (Hydrogenation)

Answer 12

nickel catalyst and 423K (150 celsius)

-breaks double bond and adds hydrogen across double bond

Question 13

Addition of halogens to alkenes (halogenation)

Answer 13

reaction works best at room temperature

Question 14

Addition of alkenes with hydrogen halides

Answer 14

room temperature
- alkene as gas is mixed with gaseous hydrogen halide/ alkene as liquid means gaseous hydrogen halide is bubbled through
- can also work if alkene is combined with concentrated HCl or HBr

Question 15

Addition of H20 (Hydration)

Answer 15

steam (H2O gas) plus phosphoric acid (H3PO4)/ Sulphuric acidd (H2SO4)
-makes an alcohol

Question 16

What is an electrophile

Answer 16

an electron pair acceptor (an atom/ group of atoms that are attracted to electron rich centres and can accept electron pairs)
-usually positive ions/molecules that contain a partial positive charge

Question 17

What is a nucleophile

Answer 17

an electron pair doner (atom/group of atoms that is attracted to an electron deficient centre, to which it can donate a pair of electrons
- usually molecules and ions with a free pair of electrons or a pi bond

Question 18

Mechanism for electrophilic addition of Hydrogen Bromide

Answer 18

1. Bromine is more electron negative than Hydrogen so HBr is polar (Bromine is delta negative and hydrogen is delta positive)
2. Electron pair in Pi bond (double bond) is attracted to partially positive hydrogen, causing the double bond to break (arrow starts from double bond and goes to delta positive hydrogen
3. HBr breaks as a bond between the hydrogen and carbon atom so a heterolytic fission occurs with the electron pair going only to bromine atom
4. Carbocation forms (a positively charged carbon atom) and Br- bromide ion forms
5. Bromide ion is attracted to the carbocation and forms the addition product

Question 19

electrophilic addition of Bromine

Answer 19

Only difference is that Br2 is non polar yet when it approaches an alkene, the pi bond electrons interact with the electrons in the Br-Br bond
–> repels electron of Br-Br, causing one side to be partially positive and the opposite side to be partially negative (induced dipole)

Question 20

Carbocation stability

Answer 20

Each alkyl group will donate and push electrons towards the positive charge of the carbocation
=> therefore, if there are more alkyl groups, the positive charge will spread throughout the molecule more and make it more stable
They can be of 3 types:
1. Primary (only 1 alkyl group)
2. Secondary (2 alkyl groups)
3. Tertiary (3 alkyl groups)

The more carbon atoms there are around the carbocation, the more stable it is

Question 21

Markownikoff’s rule

Answer 21

When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen of the hydrogen halide attaches itself to the carbon atoms of the alkene with the greater number of hydrogen atoms and smaller number of carbon atoms