2.8 Alkenes

Question 1

What is the key feature of an alkene?

Answer 1

Isolated C=C

Question 2

How many components does a alkene double bond have?

Answer 2

Two
1. Sigma framework of molecule (orbitals on the different atoms facing one another) - sp2 from each overlap
2. Pi component - overlap of 2 non-hybridised p orbitals, one on each C (high electron density above and below flat molecule)

Question 3

What is the polarity of a C=C

Answer 3

Non-polar (not free to rotate)

Sigma bonds can rotate without bonds breaking

Question 4

What are E/Z isomers?

Answer 4

E = high priority functional groups on opposite sides
Z = high priority functional groups on the same side

Question 5

What is the order of priority of functional groups?

Answer 5

C2H5 > CH3 > H & I > Br > Cl > F > C > H

Question 6

What’s the strongest force seen in molecules with just carbon carbon, and carbon hydrogen bonds ONLY?

Answer 6

Van der vaals forces
Size of molecule controls strength of molecule

Question 7

What differences are there in chemical reactivity between alkananes and alkenes?

Answer 7

• alkenes more reactive than alkanes
• alkenes tend to undergo addition reactions (particularly electrophicic addition in c=c)

Question 8

In infrared spectroscopy, what are the stretch ranges for alkene C=C and C-H?

Answer 8

C=C 1680 - 1640 cm-¹
C-H 3100 - 3000 cm-1

Not the best method for identifying alkenes. Non-polar so weak signals

Question 9

What are the ¹³C and ¹H nmr spectroscopy ranges for alkenes?

Answer 9

¹³C = 100 - 170 ppm
¹H = ~5-7 ppm (characteristic coupling)

Question 10

Which hydrogens show a small coupling constant in alkenes?

Answer 10

2 hydrogens on the same carbon - don’t easily interact with one another (germinal coupling 0-3Hz)

Question 11

Which hydrogens show a medium coupling constant in alkenes?

Answer 11

2 hydrogens which are 3 bonds apart on same side of double bond - cis coupling (6-14Hz)

Question 12

Which hydrogens show the largest coupling constant in alkenes?

Answer 12

When there’s 2 hydrogens on opposite side of double bond - very good bond arrangement - trans coupling (11-18Hz)

Question 13

What is the 1st method of alkenes synthesis?

Answer 13

By elimination of H-X from hydrogen alkanes (eg. HI, HBr)

Question 14

What reagents and conditions are needed for the 1st method of synthesising alkenes?

Answer 14

Reagent: Base eg. Sodium or potassium hydroxide

Conditions: reflux in alcoholic solution

Question 15

What is a 1 step elimination reaction involving 2 molecules described as?

Answer 15

E2 elimination
Molecularity of 2
Stereospecific mechanism - 1 to 1

Question 16

What is the 2nd type of alkene synthesis?

Answer 16

Again a dehalogenation reaction (removal of a halogen) BUT of vicinal dibromides
Molecule of Br2 removed (each on differnt carbon, 1 and 2)

Question 17

What are the reagents needed for the 2nd method of alkene synthesis?

Answer 17

• NaI
• Zn/ desolved in HOAc - acetic acid (redox reaction)

Iodide acts as a nucleophile and attacks Br

Question 18

What is the 3rd method of alkene synthesis?

Answer 18

Dehydration of alcohols - elimination of water molecule

Made easy with acids
Primary alcohol - Strong acid required
Teriary alcohol - weaker acid. Lower Ea. Less heat required

Question 19

What reagents and conditions are required to synthesise alkenes from alcohols (3rd method)?

Answer 19

Reagent: catalytic acid eg. Sulfuric or phosphoric

Conditions: Reflux (can depend on alcohols structure)

Question 20

What is the difference in temperature and reagents required when synthesising alkenes from primary and tertiary alcohols?

Answer 20

Primary: conc. H2SO4 + 180°C

Secondary: 20% H2SO4 + 85°C

Question 21

Which metabolic pathways eliminate water to make alkenes?

Answer 21

• Krebs cycle (form cis aconite acid / fumaric acid)
• Saturated hydrocarbon (chemically unreactive) > remove hydrogen as a radical > creates a cis double bond in fatty acids > alkene is formed

Question 22

Print off slide with table and learn the types of alkene addition reactions

Question 23

How many steps are involved in the electrophic addition of alkenes?

Answer 23

Two

1. Arrow from double bond (breaking it) to the slightly positive terminal of the electrophile. H of H-Cl
2. Arrow from bond in the H-Cl (for example) to the halogen. Indicating breaking
3. H, electrophile is added to the previous alkene
4. Carbocation reacts with Cl- nucleophile

Question 24

What allows for 2 products of electrphilic addition reaction?

Answer 24

Asymmetric alkene or asymmetric electrophile

Major and minor products
Discovered by Markovnikov

Question 25

What is Markovnikov’s rule?

Answer 25

H-X/water added to the double bond in a way that leads to the more stable carbocation. Major product arises from this.

This is the side of alkene with higher number of hydrogens attached to it (forms major product)

Question 26

What does adding more alkyl groups to a carbocation do to stability?

Answer 26

Increase

Teriary are most stable > secondary > primary are unstable (methyl carbocation)

Question 27

How is an alkene reduced?

Answer 27

By addition of hydrogen
H2 gas used
Heterogeneous catalyst required - usually transition metal such as Pt, Pd or Ni

Question 28

What does a transition metal catalyst do in alkene reduction?

Answer 28

It absorbs the starting material and hydrogen onto its surface.

Pulls some electron density out of the hydrogen bond and makes it more reactive

Question 29

In what form do enzymes biologically move around H2?

Answer 29

H- (comes from cofactor eg. NAD(P)H )

H+ (comes from any acidic recidues) from enzyme or water

Question 30

What recation forms diols (OH on either side) from an alkene? Reagents?

Answer 30

Dihyroxylation reaction

Reagent: osmium tetroxide (OsO4) / Cold Alkaline KMnO4

Question 31

What would hot KMnO4 do to an alkene?

Answer 31

Split it in half and form ketones/carboxylic acids

Question 32

What is used to obtain an aldehyde from an alkene?

Answer 32

Ozone (O3)

Question 33

How is the opposite distribution of major and minor products, of the addition of H-Br in Ionic reaction achieved?

Answer 33

Through radical addition

Must have an organic peroxide to begin with - Br adds before the H

Question 34

What are the 3 requirements for radical polymer formation?

Answer 34

High pressure
High temperature
Initiator

Can be difficult to control due to radical high reactivity