week 4: alkenes (1 & 2)

Question 1

what is saturation?

Answer 1

for a molecule to be saturated, it has to contain all the hydrogens it CAN contain

Question 2

what is unsaturation?
(of a hydrocarbon)

Answer 2

a hydrocarbon is unsaturated if its number of hydrogens is less than in an open-chain alkane with the same number of carbon atoms

Question 3

in comparison to an alkane, the presence of what makes a molecule unsaturated?

Answer 3

• the presence of a double bond (removes 2 H)
• the presence of a ring (removes 2 H)

Question 4

how do you work out the degrees of unsaturation (when given the chemical structure of a compound)?

Answer 4

• count the number of double bonds (each = 1 degree)
• count the number of rings
  (each = 1 degree)
• double bonds + rings

Question 5

a triple bond counts as __ degrees of saturation

Answer 5

2

Question 6

how do you work out the degrees of unsaturation (when ONLY given the chemical formula of a compound)? list the 4 cases

Answer 6

(1) pure hydrocarbon:
compare to fully saturated alkane with SAME number of carbons

(2) halogen atom(s) present:
- halogens replace H atoms
- add the number of halogen atoms to the number of H atoms
- compare to alkane with same number of carbons

(3) oxygen atom(s) present:
- insertion of an O atom does NOT change the number of H atoms
- H stays the same, compare to alkane with same number of carbons

(4) nitrogen atom(s) present:
- insertion of a N atom increases the number of H atoms by 1
- subtract one H for each N atom present
- compare to alkane with same number of carbons

Question 7

what do we do after obtaining the hydrocarbon equivalent formula of unsaturated hydrocarbons?

Answer 7

• compare with fully saturated alkane formula
• find the difference in H atoms
• divide the difference by 2 to obtain degrees of unsaturation
  (each degree = loss of 2 H)

Question 8

what is the relationship between molecule stability, its chemical reactivity, and its overall strain?

Answer 8

the more unstable a molecule is, the more chemically reactive it is, the higher strain it has

Question 9

which alkene is more stable? cis- or trans- isomers? which has more steric stain?

Answer 9

• trans- isomers are more stable, less reactive, and have no steric strain
• cis-isomers have more steric strain

Question 10

what is responsible for energy differences among disubstituted alkenes?

Answer 10

steric repulsion (or steric strain)

Question 11

how does steric strain compare in different alkane isomers?

Answer 11

• highest strain - when 2 big groups are connected to same carbon
• mid strain - when 2 big groups on separate carbons (CIS ISOMER)
• lowest strain - TRANS ISOMER

Question 12

what is a nucleophile?

Answer 12

• a molecule that has a high electron density
• is attracted to positive charge
• reacts with electrophiles

Question 13

why are alkenes considered nucleophiles?

Answer 13

the π bond in alkenes make them electron-rich (negatively charged), therefore nucleophiles

Question 14

what are attracted to alkenes?

Answer 14

electrophiles (Lewis acids)

Question 15

what is electrophilic addition?

Answer 15

a reaction where we add unit X–Y across the double bond of an alkene

Question 16

what are the steps of electrophilic addition to an alkene?

Answer 16

(1) the π (pi) bond breaks
(2) one alkene carbon gets the X, the other gets the Y of the electrophile

Question 17

in electrophilic addition reactions of HX (where X is a halogen), which reactions are more successful?

Answer 17

more likely to occur:
- HCl - used in gaseous form
- HBr - used in gaseous form

as for the rest:
- HF - too reactive
- HI - too unreactive (use KI in presence of H3PO4 (phosphoric acid) to form reaction)

Question 18

what non polar solvent is usually used to dissolve non polar alkenes in solution?

Answer 18

CCl4 (carbon tetrachloride)

Question 19

during electrophilic addition, what is the reaction intermediate called? describe it

Answer 19

• carbocation
• sp^2 hybridized
• trigonal planar geometry
• empty 2p atomic orbital

Question 20

what type of reaction is electrophilic addition? why?

Answer 20

• an exergonic, spontaneous reaction

why?
- since the first step involves breaking a pi bond, it requires energy, (the slow step)
- the second step involves the attack of a fully negative particle (ex: Br-) on the fully positive carbocation
- according to Coulomb’s law, that large electrostatic attraction makes step 2 (fast step)
- first slow, second fast = overall exergonic (-ΔG) & spontaneous

Question 21

what are the different types of electrophilic addition reactions to alkenes?

Answer 21

• orientation of electrophilic additions: Markovnikov’s rule (regiospecific)
• addition of halogens (stereospecific)
• addition of water
• hydrogenation (adding H2)
• oxidation (adding O)

Question 22

what does it mean to be regiospecifc?

Answer 22

atoms are oriented in specific positions (ex: Markovnikov’s rule)

Question 23

what does Markovnikov’s rule say?

Answer 23

in reactions of HX to an alkene, the H atom is captured by the alkene carbon attached to MOST hydrogens, and the X atom is captured by the most highly SUBSTITUTED alkene carbon (i.e the other carbon)

Question 24

which has a more stable carbocation, a methyl group (-CH3), primary, secondary, or tertiary carbocation? why?

Answer 24

• a tertiary carbocation is most stable

why?
- a methyl group donates negative electron density, reducing the positive charge on the carbocation
- however, in the tertiary carbocation, there are THREE electron-donating methyl groups, so the positive charge is much more reduced
- less positive charge = more stability

Question 25

will a primary or a tertiary carbocation produce more product after undergoing electrophilic addition? why?

Answer 25

• tertiary carbocation

why?
- even tho a primary carbocation is more reactive (bc more unstable) it has a higher activation energy
- therefore, the first step is faster with a tertiary carbocation, and will form more product first

Question 26

since elemental halogens (X2) are symmetrical (non polar), how can electrophilic addition take place?

Answer 26

• through an induced-dipole
  how?
• when for example, a Br2 molecule is far away from the alkene, it remains non polar
• when it approaches the electron-rich pi bond, its electrons are repelled, leaving one Br atom δ+, and the more distant Br atom δ- (creating an induced-dipole)
• the bromine molecules become polarized

Question 27

what does it mean to be stereospecific?

Answer 27

atoms go in particular 3-dimensional orientations (ex: halogens added to alkenes)

Question 28

explain how the addition of halogens to alkenes is a stereospecific reaction?

Answer 28

• when adding Br2 to an alkene ring, we will have a trans molecule
• this is because halogens are big molecules, and if they were captured on the same side, they would repel each other
• the driving force of this is steric strain - (there will less steric strain in a trans form)

Question 29

what is the addition of water to alkenes reaction called? what does it produce?

Answer 29

• reaction is called hydration
• forms alcohols

Question 30

what compounds will NOT get reduced by hydrogenation?

Answer 30

• benzene rings
• carbonyl groups

Question 31

what are the different methods of adding water to alkenes (or hydration reactions)?

Answer 31

(1) brute-force (Markovnikov addition)
(2) oxymercuration (Markovnikov addition)
(3) hydroboration (anti-Markovnikov addition)

Question 32

describe brute-force reactions

Answer 32

• typical Markovnikov rules, where you add the H to the carbon attached to most hydrogens, and the OH to the other carbon
• the reaction is acid-catalyzed because the H+ atom from the acid is used to break the pi-bond

Question 33

what problems do brute-force reactions have?

Answer 33

• highly acidic conditions (50% sulfuric acid)
• need high temperatures
• NOT ideal for research/undergrad lab environments

Question 34

what are the steps of oxymercuration? does it follow Markovnikov or anti-Markovnikov addition?

Answer 34

• step 1: treatment with mercuric acetate ( Hg(OAc)2 )
• step 2: treatment with sodium borohydride (NaBH4)
• follows Markovnikov addition

Question 35

what are the steps of hydroboration? does it follow Markovnikov or anti-Markovnikov addition?

Answer 35

• step 1: addition of borane (BH3) to diethyl ether (Et2O)
• step 2: addition of H2O2 in NaOH solution
• follows anti-Markovnikov addition

Question 36

what is meant by syn-addition in hydroboration?

Answer 36

if we have a ring alkene, the -H and -OH are added on the same side of the ring (cis)

Question 37

what type of reaction is alkene hydrogenation?

Answer 37

a reduction