Chapter 7: Alkenes

Question 1

What is an Alkene?

Answer 1

Groups with c-c pi bonds.

Question 2

What is the basic formula of an alkene?

Answer 2

C_NH_2N

2 less hydrogens than alkanes C_NH_{2N + 2} which makes alkenes unsaturated because they do not have the max amount of H on the molecule

Question 3

What is the unsaturation or double bond equivalent (DBE) formula and what does it help us calculate?

Answer 3

DBE= 2C + 2 - H - x + N/2

Helps calculate the number of pi bonds or rings in a molecule which helps solve for structure (will be important in spectroscopy)

Question 4

What are double bonds?

Answer 4

Double bonds are 2 carbons that share a sigma and pi bond.

Question 5

IUPAC rules for Alkenes

Answer 5

1. Alkene demands priority so it has to be on the lowest number of carbons regardless of substituents
2. Find the largest continuous carbon chain that has the alkene (ring or chain)
3. Drop the -ane and add -ene

Question 6

Diastereomers of Alkenes

Answer 6

1. cis/trans disubstituted alkenes
2. E/Z designation for alkenes

Question 7

How to designate E/Z for alkenes

Answer 7

• Use Cahn-Ingold-Prelog rules of priority
• # 1 falls on the same side then (Z)
• # 1 falls on the opposite side then (E)

Question 8

True or False: An alkene will not have E or Z if each carbon in c-c pi bond is attached to 2 different things.

Answer 8

False.

Question 9

What is the vinylic position on an alkene?

Answer 9

An atom or substituent is in the vinylic position when they are directly attached to an alkene.

Question 10

What is the allylic position on an alkene?

Answer 10

A group is in the allylic position on an alkene when it is next to the alkene.

Question 11

How to determine how much energy is in an alkene

Answer 11

Use bond energies to calculate delta H^o_rxn

Question 12

How to calculate delta H^o_rxn of an alkene H-X addition mechanism

Answer 12

delta H^o_rxn = (sum H^o_{bonds broken)} - (sum H^o_{bonds formed)}

Question 13

What is the sign of delta H^o of alkene mechanisms?

Answer 13

(-) exothermic. Energy occurs on the product side of rxn because alkenes will give off energy if you reduce or hydrogenate it.

Question 14

What are the three types of alkenes?

Answer 14

Mono substituted, disubstituted, and trisubstituted. Substitution matters

Question 15

Which type of alkene is the most stable? Monosubstituted, disubstituted, and trisubstituted.

Answer 15

trisub > disub > monosub

Trisubstituted is the most stable, Monosubstituted is the least stable

Question 16

Why is a trisub alkene the most stable compared to disub (middle) and monosub (least)?

Answer 16

More substituted alkenes are more thermodynamically stable.

Giving off less heat (less energy) creates more stability. Giving off more heat (more energy) creates less stability.

Question 17

Are cis or trans alkenes more stable?

Answer 17

Trans are more stable because it gives off less heat and is lower in energy than cis due to avoidance of steric strain.

Question 18

What is steric strain?

Answer 18

Cis alkenes create very close electron clouds called steric strain (electrons bumping into each other).

Steric strain adds energy to a system making it less stable.

Question 19

What is a carbocation?

Answer 19

A carbon that only has 6 electrons in its valence shell and is covalently bonded to 3 atoms.

Carbon can have 6 electrons because it is not super electronegative.

Question 20

What are the classes of carbocations?

Answer 20

⁺CH₃ - methyl (C⁺ nothing attached)

1^o primary (1 Carbon attached)

2^o secondary (2 Carbons attached)

3^o tertiary (3 Carbons attached)

Question 21

What is the trend for the stability of carbocations?

Answer 21

3 >> 2 > 1

more substitution=more stable or lower in energy

Question 22

What is the origin of carbocation stability?

Answer 22

Hyperconjugation. Electron delocalization called hyperconjugation which is an overlap of a pair of electrons between a filled and empty orbital that can’t be described by resonance.

Question 23

What is the trend between hyperconjugation and stability?

Answer 23

More hyperconjugation=more stable

The more bonds that you have next to a carbocation the more hyperconjugation. The more occurrences of hyperconjugation, the more stable the carbocation.

Question 24

What is the name of the Alkene reaction mechanism?

Answer 24

H-X Addition

Question 25

H-X addition

Answer 25

Alkene is a loose sea of electrons that reacts with a strong acid to break the bond from halide

Question 26

Electrophile

Answer 26

anything trying to accept electrons; electron hungry

Question 27

nucleophile

Answer 27

anything that gives its electrons to a nucleus; nucleus hungry

Question 28

Reactions occur on the lowest energy pathway which helps chemists

Answer 28

predict the product

Question 29

What is a coordinate diagram or potential energy surface (PES)?

Answer 29

A diagram indicating the differences in the energy of global minima (starting material/products) and reaction intermediates. We use the differences in energy pathways to predict products.

Question 30

On the PES, what does the number of transition states depend on?

Answer 30

The number of transition states depends on how many arrows are drawn in the reaction.

Question 31

On the PES, what does the higher TS peak show?

Answer 31

The highest TS peak (usually peak 1) is the RDS (slow step) meaning that the rxn can proceed. TS from starting materials (SM) will be higher than the TS to products.

Question 32

On the PES, how are the energy of the SM and intermediate related?

Answer 32

The energy of the rxn intermediate will always be higher than the SM and product or else the rxn will not proceed

Question 33

On PES, what is the term to define the difference in energy between the highest TS and SM?

Answer 33

Activation Energy (E_A)

Question 34

On PES, where do we find the delta H^orxn?

Answer 34

Delta H^orxn=SM-products

Question 35

What does Markovnikov’s rule state?

Answer 35

Proton goes on the carbon w/ more protons during the first step of an alkene mechanism (predicts regioselectivity)

Question 36

What is regioselectivity?

Answer 36

Regioselectivity is when one constitutional isomer is formed over all others. Markovnikov’s rule predicts this by choosing the Carbon with the most hydrogens attached during RDS.

Question 37

An alkene reaction always wants more hydrogens and higher carbocation class to

Answer 37

stabilize and speed up a reaction to make a major product

Question 38

What is Hammond’s postulate?

Answer 38

TS resembles the thing closest to energy to it.

Question 39

Hammond’s rule says that the TS from SM to 3^o carbocation will be lower in energy than the TS from SM to 1^o carbocation.

The faster the path, the lower the

Answer 39

energy

Question 40

What happens when Mark’s rule or more H of carbons does not exist?

Answer 40

Same # H = same rxn intermediate energy. So, products will be a mix 50/50 and both paths create the same carbocation (i.e. 1,2,3)

Question 41

What are the 3 types of Carbocation rearrangements or types of shifts of alkenes (wagner-meerwein shifts)?

Answer 41

1. 1,2 - hydride shifts
2. 1,2 - methyl shifts
3. Ring expansion

Question 42

What is a 1,2 - hydride shift?

Answer 42

C-H bond donates too much electron density or energy and it moves over, represented by a curvey arrow. Hydride shifts are faster than the halides attacking the alkene.

Question 43

What is a 1,2 - methyl shift?

Answer 43

Methyl group on neighboring carbon donates its electrons and shifts to the carbocation

Question 44

What is a ring expansion shift and why are they important?

Answer 44

Ring expansion stabilizes the product by creating a higher carbocation class and decreasing ring strain. Rings have ring strain due to a lack of 109.5-degree bond angles and eclipse conformations.

Question 45

Shifts do not always happen. Shifts can only happen with

Answer 45

an increase in carbocation class or a decrease in ring strain (by expansion).