2.2

Question 1

Regioselective

Answer 1

Reaction prefers to form changes in one particular region.

Question 2

Sterioselective/stereospecific

Answer 2

Only one stereoisomer formed

Question 3

What is the criterion for an alkene to have cis and trans isomers?

Answer 3

Each end of the alkene having a pair of two different groups attached.

Question 4

In a hydration reaction, does the alkene act as a nucleophile or as an electrophile?

Answer 4

Nucleophile.
Alkenes have a high electron density (there are 4 electrons in their double bond), and the donation of the 2 pi electrons allows them to act as a nucleophile.

Question 5

How do electrons move in a alkene addition reaction?

Answer 5

electrons move from the nucleophile to the electrophile

Question 6

What is an electrophile?

Answer 6

An electrophile is electron poor (low electron density).

- accepts electrons from nucleophile

Question 7

what is a nucleophile?

Answer 7

a nucleophile is electron rich (high electron density)

- donates electrons to nucleophile.

Question 8

Why are alkenes electron rich?

Answer 8

C-C double bond in alkenes consist of one strong sigma bond due to head on overlapping of sp^2 hybridised Orbital and one weak pi bond obtained by lateral or sideways overlapping of the two 2p of the two carbon atoms. The double bond is shorter in bond length than the C-C single bond.

The pi bond is a weaker bond due to poor sideways overlapping between the two 2p orbitals. Thus,the presence of the pi bond makes alkenes behave as sources of loosely held mobile electrons.
The presence of weaker pi bond makes them unstable in comparison to alkanes.
Hence alkenes are are electron rich.

Question 9

What is the intermediate process of an alkene addition reaction?

Answer 9

When the cation and the anion separates the cation migrates to the alkene. Electrons in the pi bond are held weakly and are donated to the cation to stabilize its positive charge. This results in

Protonation occurs on the least substituted carbon as this means that a more stable carbocation will form.

The H+ ion has no electrons, so the two electrons involved in the pi bond both move to the positive H+ ion: electrostatic attraction. This means that of the two carbons involved in the double bond which had each 1 electron involved in bond, the least substituted carbon now has one tied up in a bond with hydrogen and the other one has simply one less: leading to the formation of a carbocation. This carbocation then draws the anion due to electrostatic force, which after nucleophilic attack, stabilizes it by donating an electron.

Question 10

What is the first transition state of an alkene addition reaction?

Answer 10

First: protonation occurs as proton acts as electrophile and alkene acts as nucleophile, and 2e- are donated from the double bond to the electrophile.

This leaves a carbocation and an anion.

Question 11

What’s the second transition state of an alkene addition reaction?

Answer 11

To stabilize the the carbocation the anion donates one electron, acting as a nucleophile, to the carbon group containing carbocation which acts as an electrophile.

This leaves a stable product.

Question 12

What is Markovinov’s Rule?

Answer 12

Markovinov’s Rule says that protonation occurs on the least substituted carbon.

This gives the most stable carbocation intermediate, as the more carbons around the carbocation the greater potential for electron density to be donated from these carbons to the carbocation thus leading to stabilization of the molecule.

Question 13

what are the energetics of an alkene addition reaction?

Answer 13

Potential energy peaks during the first transition state and dips as the carbocation and anion are formed, increasing as the second transition occurs and then falls away.

Question 14

Explain the energetics of an alkene addition reaction

Answer 14

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Question 15

What is the relationship between energy and stability?

Answer 15

Lower energy = greater stability

So more substituted carbons are lower in energy meaning they are more stable.

Question 16

What is necessary for the addition of H2O to an alkene?

Answer 16

This is an acid catalyzed reaction because the H+ technically come off the HCL but the acid is regenerated by the deprotonation of the alcohol so its technically a catalyst.

Question 17

Why is addition of halogens to alkenes stereoselective?

Answer 17

Because as the reaction occurs in two steps a bromonium intermediate ion forms which blocks one side of the molecule from nucleophilic attack so the other bromine bonds to the carbocation on the other side of the molecule leading to formation of a trans isomer exclusively.

Question 18

How does hydrogenation of alkenes occur with the addition of H2?

Answer 18

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Question 19

Is the hydrogenation of alkenes stereoselective?

Answer 19

Yes, stereoselective. Forms cis isomers.

Question 20

Why is the hydrogenation of alkenes stereoselective?

Answer 20

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Question 21

What is a use of hydrogenation of alkenes?

Answer 21

• Used to create alkenes into alkanes, used in food industry for production/synthesis of margarine.

Question 22

How does the metallic catalytic hydrogenation of alkenes on a metal surface work?

Answer 22

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Question 23

what do these mean?
saturated fats
unsaturated fats
polyunsaturated fats

Answer 23

Saturated: alkane
Unsaturated: double or triple bonds
Polyunsaturated: multiple double or triple bonds.

Question 24

Fat vs an oil?

Answer 24

• In a fat all the backbones are saturated so molecule forms a tidy uniform shape that packs together
• in an oil double bonds make the chains go bendy everywhere so molecules don’t pack well and slide past each other.

Question 25

What’s a Lindlar catalyst?

Answer 25

A doped Pd (lead) catalyst that gives a cis-alkene as the product of an alkyne addition reaction and only reacts once.

Question 26

What is required in the addition of alkyne reactions?

Answer 26

Alkyne addition reactions usually require a catalyst and usually form and alkane.

Question 27

Why are terminal alkynes special?

Answer 27

Terminal alkynes have an acidic hydrogen.

This means deprotonation with a strong base gives a good nucleophile.

This good nucleophile (good at donating electrons) can be used to undergo substitution to create an internal alkyne to lengthen chains.

Question 28

How are aromatic addition reactions different?

Answer 28

• After addition, elimination occurs or deprotonation, so that the product is formed and a H+ ion is ejected because to incorporate both new substituent and old H atom would require breaking of double bond and loss of aromaticity.

The result is a substitution reaction where aromaticity is retained.

Question 29

What is the resonance energy?

Answer 29

The resonance energy is the difference in energy between a resonance hybrid and its most stable hypothetical contributing structure.