A3 Alkenes

Question 1

how are alkene carbons bonded together

Answer 1

connected by a sigma bond based on sp2 hybridised orbitals

Question 2

how are alkyne carbons bonded together

Answer 2

connected by a sigma bond based on sp hybridised orbitals

Question 3

what happens to the rest of the orbitals which are not involved in bonding of carbon atoms

Answer 3

can overlap in phase to give pi bonding orbital or out of phase to give pi* antibonding orbital

Question 4

which orbitals are involved in the reaction when alkenes are alkyls act as nucleophiles

Answer 4

pi orbitals with the highest occupied molecular orbitals

Question 5

draw alkene orbitals in phase and out of phase

Question 6

draw alkyne orbitals in phase and out of phase

Question 7

what occurs when hydrogen reacts with an 1-2,dimethyl-cyclohexene and what conditions are required

Answer 7

undergoes cis-addition to form 1-2,dimethylcyclohexane
requires Pt catalyst

Question 8

what occurs when Br2 reacts with 1-2,dimethyl-cyclohexene

Answer 8

undergoes trans addition to form 1-2 bromo, 1-2 methyl hexane

Question 9

what occurs when HX reacts with 1-2,dimethyl-cyclohexene

Answer 9

cis and trans addition occurs forming 1-bromo 1-2 methyl cyclohexane

Question 10

why hydrogenation of alkenes for cis compounds

Answer 10

because carbon atoms approach the catalyst surface at the same time so the H get attached onto the same face of the alkene

Question 11

how can alkynes undergo hydrogenation (without a Pt catalyst)

Answer 11

Lindlars catalyst (Pd + BaSO4 CaCO3)

Question 12

is it easier going from alkene to alkane or alkyne to alkene….explain

Answer 12

alkyne to alkene because it is possible to stop the reaction from proceeding through the 2nd reaction it must be harder. This is because when there are multiple pi orbitals the greater electron density makes the alkyl less stable so more reactive

Question 13

draw the mechanism for bromination of alkenes

Question 14

why can only trans cyclicbromo alkanes form

Answer 14

due to steric hinderance

Question 15

what is a stereospecific reaction

Answer 15

a reaction which produces a single stereoisomer

Question 16

draw the mechanism for bromination of a trans (E) alkene

Question 17

draw the mechanism for the bromination of a cis (Z) alkene

Question 18

why are bromohydrins useful in synthesis routes

Answer 18

provide a synthetic route to epoxides

Question 19

draw the mechanism for the formation of transbromo hydrin

Question 20

how can transbromo hydrin be coverted to an epoxide

Answer 20

treated with a strong base Na+ H+ to form an intramolecular Sn2 reaction

Question 21

are shorter bonds weaker or stronger than regular bonds

Answer 21

stronger

Question 22

are longer bonds stronger or weaker than regular bonds

Answer 22

weaker

Question 23

draw a mechanism for the bromination of 2-methyl propene (and the preferred product…explain)

Answer 23

this is preferred as the longer bond in the transition state is easier to break as it is more polarised and has the most electrophilic end

Question 24

draw the mechanism for synthesis of epoxides from metacholoperoxybenzoic acid and but2ene

Question 25

what is a percid and why are the good electrophiles

Answer 25

a carboxylic acid with an extra oxygen in between the electrophilic O-O bond is high energy

Question 26

put regular, primary, secondary and tertiary alkenes in order of relative rates of reaction

Answer 26

regular

Question 27

why are tertiary alkenes the most reactive

Answer 27

because they enhance the reaction and don't cause steric hinderance like other tertiary molecules because the molecule is flat

Question 28

what is Markovnikov's rule and explain it

Answer 28

addition of HX to an unsymmetrical system alkene gives the more highly substituted product because the most stable cation intermediate gives the most substituted product (lowest energy transition state)

Question 29

what is hydration

Answer 29

addition of water to an alkene

Question 30

do alkenes favour hydration or dehydration entropically and why

Answer 30

dehydration because it increases disorder by splitting one molecule into 2

Question 31

what reaction does not fit Markovnikov's rule

Answer 31

hydroboration as the less substituted alcohol product is formed

Question 32

draw the mechanism for hydroboration of methylcyclohexene

Question 33

what type of reaction is the hydrobroation of methylcyclohexene

Answer 33

radioselective stereospecific cis-addition

Question 34

what are the reagents required for the electrophillic aromatic substitution of bromine and benzene

Answer 34

Br2 FeBr3 (Lewis Acid catalyst)

Question 35

what is the structural proof that benzene isnt an alkene

Answer 35

all bonds are the same length (not alternating)

Question 36

why do benzene rings have their own current and how does that effect its chemical shift

Answer 36

the movement of electrons flowing around the ring creates a current that Deshields the protons to a higher chemical shift

Question 37

what is the thermodynamic evidence for benzene ring

Answer 37

in theory is cyclohexyltriene bonds were broken 360kjmol-1 would be released but for benzene only 152kjmol-1 are released

Question 38

what are the reagents needed to form cyclohexane from benzene

Answer 38

H2, high temperature, high pressure and a strong catalyst

Question 39

does cyclooctetraene have a planar structure

Answer 39

no but can be made planar via oxidation to a dication otherwise it has a bowl shape conformation

Question 40

what is huckels rule

Answer 40

planar, fully conjugated rings with 4n+2 pi electrons and no unpaired or antibonding electrons (closed shell) are aromatic planar fully conjugated rings with 4n pi electrons and some unpaired (open shell) in antibonding orbitals are anti-aromatic

Question 41

draw an electron configuration for an open shell and closed shell ring