reactivity at sp centres +

Question 1

Do alkenes or alkynes have lower melting boiling points and why?

Answer 1

Alkenes have lower melting points due to their less efficient packing structure

Question 2

Are alkynes acidic or basic and why?

Answer 2

Acidic because of the sp nature of the alkynes (greater s character) makes carbon slightly more electronegative which makes it better at carrying the negative charge (BUT still requires a STRONG BASE)

Question 3

Why are sp carbons slightly more electronegative?

Answer 3

Greater s character means that the electrons are closer to the nucleus

Question 4

What type of alkyne must it be for a reaction to occur?

Answer 4

terminal alkyne

Question 5

What is the step by step of hydrohalide reactions?

Answer 5

alkyne + hydrogen halide -> E/Z mixture of alkenyl halide

alkenyl halide + hydrogen halide -> geminal dihalide

Question 6

What is a geminal halide?

Answer 6

halides are attached to the same carbon

Question 7

With alkyne reactions, what is the intermediate and can it be isolated?

Answer 7

Alkene intermediate that reacts very quickly, cannot be isolated

Question 8

Why do geminal dihalides form instead of one halide on each carbon for alkyne reaction with hydrogen haldie?

Answer 8

The halide has lone pairs which can form resonance structures to stabilize the negative charge (carry the positive) thus the carbon would attached to the halide would be happier to carry the positive charge meaning that it will be attacked by the nucleophile

Question 9

What are the conditions required to hydrate alkynes and what is formed?

Answer 9

water, acidic conditions and high temperatures and ketone is formed

Question 10

Draw out the mechanism for the hydration of alkyne to form the ketone

Answer 10

Lecture 9 p5 for answer

Question 11

What are the actual (milder) conditions to create stable ketones?

Answer 11

Water, gold catalyst (NaAuCl4) and 70 degrees C

Question 12

What are the 2 ways to stop an alkyne reaction at an alkene and which stereoisomer does it form?

Answer 12

1. Lindlar catalyst (palladium poisoned with lead acetate and quinoline)
   - poisoned palladium selectively hydrogenates alkynes and not alkenes (if not poisoned reaction finishes at alkane instead)
   - Z alkene formed
2. metallic Na to ammonia to form E alkene then addition of tBuOH to get the H to form alkene
   - E alkene formed

Question 13

Why does the poisoned palladium always form Z stereoisomer?

Answer 13

Hydrogens are collected from the same side thus end up on the same face (interface between solid and liquid)

Question 14

How to form alkane from alkyne?

Answer 14

UNPOISONED normal palladium catalyst with H2

Question 15

What is the mechanism for Z alkene formation from alkyne?

Answer 15

1. Na releases electrons to form Na+
   2.Alkyne accepts electron and one becomes a radical and one becomes negatively charged (BOTH FISH HOOK ARROWS)
2. tBuOH donates H x 2 with electron donation to free radical to form negative charge inbetween

Question 16

What colour does the ammonia solution turn with solvated electrons?

Answer 16

Deep blue

Question 17

What shape does the radial anion adopt and why?

Answer 17

Trans geometry (E isomer) to maximise electron-electron repulsions (as far away apart as possible)

Question 18

what is meant by antimarkovnikov product?

Answer 18

hydrogen is attached to the carbon with less hydrogens on it

Question 19

What are the unique properties of BH3 and what does it form?

Answer 19

has an empty p orbital and the BH3 tautomerizes to form B2H6 (diborane)

Question 20

What is the regioselectivity of addition of borane to alkenes and why?

Answer 20

BH2 is always added to the less substituted carbon due to partial formation of slightly positive carbocation that needs to be stabilized through hyperconjugation

Question 21

what is meant by a concerted reaction?

Answer 21

bonds and forming and breaking at the same time (but can be at different rates). a single step, no intermediate reaction

Question 22

what type of transition state is formed by reaction of borane and alkene

Answer 22

4 membered transition state with partial positive and negative charge (see L10 p6)

Question 23

what is the stereoselectivity of hydroboration and why?

Answer 23

as it is a concerted reaction the substituents are added to the same side, thus is a syn addition reaction

Question 24

what are 2 ways to extend C-C bonds with alkynes?

Answer 24

adding a alkyl halide or an epoxide

Question 25

what are the 2 things needed for alkyne reactions?

Answer 25

a strong base and an electrophile

Question 26

what are the 2 things needed for diels-alder reaction?

Answer 26

alkene with an EWG next to it
conjugated diene

*HOMO of the diene is close in energy to the LUMO of the alkene

Question 27

what conformation must the diene be in for the diels-alder reaction?

Answer 27

cis

Question 28

what is the stereospecificity of the diels-alder reaction?

Answer 28

maintains the stereochemistry of the initial molecule