Isomerism + Alkenes

Question 1

What is geometric isomerism?

Answer 1

cis-trans isomerism
cis - Z (highest priority groups on the same side)
trans - E (highest priority groups on opposite sides)

it is caused by a lack of rotation around double bonds
- rotation around double and triple bonds would results in the sideways overlap p orbitals breaking

have the different chemical and physical properties

Question 2

What are isomers?

Answer 2

molecules that have the same molecular formula but have a different arrangement of atoms in space

Question 3

What is stereoisomerism?

Answer 3

molecules which are non-superimposable mirrors images of one another

• R and S enantiomer = interchanging any two groups at the chiral centre switches between the enantiomers
• have a chiral carbon = 4 different constituents, asymmetrical

R - clockwise
S - anticlockwise

Question 4

What are the properties of chiral molecules?

Answer 4

have identical melting and boiling points
have identical solubilises in solvents
have identical spectra and refractive index
interact and react with chiral molecules in the same manner

interact and react with other chiral molecules at different rates
rotate plane polarised light in opposite directions but equal amounts (to the same degree) = are optically active

Question 5

How do you calculate enantiomeric excess?

Answer 5

%ee = (observed specific rotation/ pure specific rotation) x 100

Question 6

What is a racemic mixture?

Answer 6

a mixture contains equal amounts of both the R and S enantiomers
- consist of chiral molecules but does not have optical activity

Question 7

What is a chiral centre?

Answer 7

chiral centres are tetrahedral atoms, sp3 atom, which have four different groups, ligands, bonded to it. These four different groups mean that the molecule isasymmetricaland non-superimposable on its mirror image.A molecule with a chiral centre has two enantiomers, R and S, which are stereoisomers.

Question 8

What is the difference between alkanes and alkenes?

Answer 8

alkane

• saturated hydrocarbon
• contains only single bonds

alkenes

• unsaturated hydrocarbon
• contains double bonds

Question 9

What is the test for unsaturated compounds?

Answer 9

add bromine water a sample
alkanes
- does not react with bromine, does not decolourise the solution
- orange-brown solution

alkenes

• react with the bromine, decolourise the solution
• colourless solution

alkynes

• react with the bromine, decolourise the solution
• colourless solution

Question 10

How does bromine react with double bonds?

Answer 10

double bond and bromine attack each other with double headed arrows. bond attacks bromine

one bromine forms a three membered ring with the carbon while the other becomes am octet.

octet attacks carbon and forms alkane

Question 11

How can a cycloalkane be in the trans isomerism?

Answer 11

both bonds will be diaxial or diequitorial

Question 12

What is a carbocation?

Answer 12

positively charged carbon

• sp2 hybridised = 3 sigma bonds and 1 empty p orbital
• trigonal planar = 120

Question 13

What is Markovnikov’s Rule?

Answer 13

addition of HX to an unsymmetrical alkene

- protons must always join the carbon with the greatest number of H atoms to form the major product

Question 14

What is an addition reaction? What is the slow step in an addition reaction onto alkenes?

Answer 14

addition reaction

• atoms are added to a bond
• alkenes form alkanes

the slow step is the breaking of the pi bond
- first reaction with HX

Question 15

What reactions do alkanes, alkenes and alkynes go under?

Answer 15

alkanes = substitution reactions

alkenes, alkynes = addition reaction (electrophilic)

Question 16

What compounds can be resonance stabilised?

Answer 16

allylic carbocation

benzylic carbocation

Question 17

What are the types of additions reactions that alkenes can go through? What do they form?

Answer 17

alkenes can become alkyl halides 
- react with HX = hydrogen halide
alkenes can become alkyl hydrogen sulphide 
- react with sulphuric acid (H2SO4)
alkenes can become alcohols 
- react with sulphuric acid then water and heat 
alkenes can become dihaloalkanes
- react with dihaloalkanes

Question 18

What is the order of stability in carbocations?

Answer 18

tertiary carbocation - most stable
= persistent, most likely to form in solution
secondary carbocation - second most stable
primary carbocation - third least stable
methyl carbocation - most unstable