Alkenes✅

Question 1

How is a pie bond formed

Answer 1

By the sideways overlap of two p-orbitals, one from each C atom of the double bond

Question 2

Where is the electron density of the pie bond, what does the pie bond also do in regards to rotation

Answer 2

Concentrated above and below the line joining the nuclei of the bonding atoms

Pie bond locks the 2 C atoms in position and prevents them from rotating around the double bond

Question 3

What is the bond angle in an alkene

Answer 3

120

Question 4

State what is meant by unsaturated (1 mark)

Answer 4

Compound with one or more double or triple bonds

Question 5

Explain the difference between a sigma and pie bond (3 marks)

Answer 5

A sigma bond is result of the head on overlap of orbitals whereas a lie bond is formed from the sideways overlap of p orbitals

Question 6

Why is the bond angle around the C atom in an alkene different to an alkane (3 marks)

Answer 6

In alkene there are 3 bonding regions about the C atom, these repel equally resulting in trigonal planar. In alkane 4 bonding pairs of electrons, these repel equally in a tetrahedral shape, trig planar is 120, tetrahedral is 109.5

Question 7

What is a stereoisomer

Answer 7

Same structural formula but different arrangement of the atoms in space

Definition: molecules with the same structural formula but a different arrangement of the atoms in space

Question 8

When will a E/Z isomer occur

Answer 8

A C=C double bond
Different groups attached to each C atom of the double bond

Question 9

What is the difference between a E and Z isomer

Answer 9

E, if the groups of higher priority are diagonally placed across the double bond
Z, group of higher priority are in same side of the double bond

Question 10

What does cis and trans isomer equate to in terms of E/Z isomer

Answer 10

Cis=Z
Trans=E

Question 11

What determines the priority in an E/Z isomer

Answer 11

Atomic number is bigger has a higher priority if it’s the same it’s what comes after it

Question 12

Why are alkenes much more reactive than alkanes

Answer 12

Presence of pie bond, the pie electrons are more exposed than sigma bond electrons, a pie bond readily breaks and alkenes undergo addition reactions relatively easily

Question 13

What is the bond enthalpy of a sigma and pie bond

Answer 13

Sigma is 347KJmol-1
Pie is 265kJmol-1

Question 14

Give examples of alkenes that undergo addition reactions with

Answer 14

Hydrogen in presence of nickel catalyst
Halogens
Hydrogen halides
Steam in presence of acid catalyst

Question 15

What conditions does there need to be for hydrogenation of alkenes, give an example with propene

Answer 15

423K, passed over a nickel catalyst
Propene becomes propane

Question 16

Give an example with propene and bromine for the halogenation of alkenes

Answer 16

Propene + bromine= 1,2-dibromopropane

Question 17

How do you test for unsaturation

Answer 17

Bromine water added dropwise to sample the oranje color disappears as double bone broken

Question 18

Conditions for addition reactions of alkenes with hydrogen halides give an example of propene with hydrogen chloride

Answer 18

Alkenes react with gaseous hydrogen halides at room temp, if alkene is a gas reaction takes place when 2 gases mixed, if alkene liquid hydrogen halide bubbles through

Propene + HCl= 1-chloropropane or 2-chloropropane

Question 19

What are the conditions and give an example of propene with hydration reactions of alkenes

Answer 19

Alcohols formed when alkenes react with steam in presence of a phosphoric acid catalyst

Propene + steam under presence of phosphoric acid catalyst forms propan-1-ol or propan-2-ol

Question 20

Why is the mechanism called electrophilic addition

Answer 20

Double bond in alkene represents region of high electron density cause of presence of pie bond electrons

High electron density of pie electrons attract electrophiles (atom or group of atoms attracted to an electron rich center and accepts an electron pair)

Question 21

Describe the electrophilic addition mechanism with bromine and but-2-ene

Answer 21

1: br more electronegative than hydrogen, so H bromide is polar and contains dipole of H+ and br -
2: electron pair in our hind attracted to partially positive H cause double bond to break
3: bond forms between H atom of H-Br molecule and a C atom that was part of double bond
4: the H-Br bond breaks by heterolytic fission, with electron pair going to bromine atom

Question 22

What is the most stable carbocation

Answer 22

Tertiary, secondary, primary

Question 23

What determines the major product

Answer 23

Carbocation stability

Question 24

What is a repeat unit

Answer 24

The specific arrangement of atoms in polymer molecule that repeats over and over again
Written in square brackets with n bottom right corner

Question 25

How is poly(ethene) forked

Answer 25

Heating large number of ethene monomers at high pressure

Question 26

What makes polymers suitable for storing food and chemicals safely

Answer 26

Lack of reactivity

Question 27

How do you get rid of polymers

Answer 27

Recycling, using waste polymers as fuel, feedstock recycling, biodegradable polymers, photodegradable polymers,

Question 28

NEED TO DO PAST PAPERS ON GETTING RID OF POLYMERS + MECHANISM

Question 29

what isomerism is E/Z describe the difference

Answer 29

stereoisomerism

E= higher priority groups on opposites sides

Z= higher priority groups on same side

Question 30

when is ciz/trans isomerism used

Answer 30

2 of the groups attached to each C atoms in the double bond are the same

H atom is attached to C in double bond

Cis=Z
trans=E