Ch. 9 - Alkynes Basics

Question 1

geminal dihalide

Answer 1

Two groups connected to the same carbon atom. For example, a geminal dihalide is a compound with two halogens connected to the same carbon atom.

Question 2

Vincinal Dihalide

Answer 2

A term used to describe two identical groups attached to adjacent carbon atoms.

Question 3

why is Sp >sp2

Answer 3

…

Question 4

1. xsNaNH2/NH3
2. H2O

Answer 4

a terminal alkyne can be prepared by treating a dihalide with excess (xs) sodium amide followed by water

Dihalide ——> terminal alkyne

Question 5

To produce an alkane, an alkyne can be treated with

Answer 5

H2 in the presence of a metal catalyst, such as Pt, Pd, or Ni.

Triple bond to single bond

Question 6

poison catalyst

Answer 6

a partially deactivated catalyst, called a poisoned catalyst, it is possible to convert an alkyne into a cis alkene (without further reduction):

Question 7

dissolving metal reduction

Answer 7

a reaction in which an internal alkyne ( R- triple bond - R) is converted into a trans alkene

reagents: sodium metal (Na) in liquid ammonia (NH3)

1. Na
2. NH3 (l)

Electron transfer e-
proton Transfer H+
Electron transfer e-
Proton Transfer H+

Question 8

To produce a cis alkene, an alkyne can be treated with

Answer 8

(triple bond to cis double bond)

H2 in the presence of a poisoned catalyst, such as Lindlar’s catalyst or Ni2B.

Question 9

To produce a trans alkene, an alkyne can be treated with

Answer 9

( triple bond -> trans double bond)
sodium metal in liquid ammonia.

1. Na
2. NH3(l)

Question 10

How to convert a cis alkene (double bond) into a trans alkAne ( single bond)?

Answer 10

1. H2
2. Pt ( or Pd or Ni)

Question 11

How to convert a Trans alkene (double bond) into a trans alkAne ( single bond)?

Answer 11

1. H2
2. Pt ( or Pd or Ni)

Question 12

Halogenation of Alkynes
What does the addition of
HX do to a triple bond vs (xs) HX

Answer 12

triple ——HX—-> double
triple ——(xs)HX—-> single

Question 13

describe the addition of HX for alkynes

Answer 13

Markovnikov addition is observed when alkynes are treated with HX
the halogen is installed at the more substituted position.

When the starting alkyne is treated with excess HX, two successive addition reactions occur, producing a geminal dihalide

Question 14

describe the mechanism for the addiiton of (xs) HX

Answer 14

1. proton trasnfer producing an intermediate carbocation ( vinylic carbocation)

2.nucleophilic attack of the carbocation ion

3. ???

Question 15

How does the addition of a peroxide affect the addition of HX

Answer 15

HBr undergoes an anti-Markovnikov addition across an alkene

The Br is installed at the less substituted position, and the reaction is believed to proceed via a radical mechanism

The Br is installed at the terminal position, producing a mixture of E and Z isomers

Radical addition only occurs with HBr (not with HCl or HI)

Question 16

How to change a dihalide to an alkyne

Answer 16

1. xs NaNH2/ NH3
2. H2O

Question 17

Acid-catalyzed Hydration of Alkynes

Answer 17

Alkynes are also observed to undergo acid-catalyzed hydration (via a Markovnikov addition)
Reagent:
1. H2SO4, H2O
2. HgSO4
generates a melthyl ketone

but the reaction is slower than the corresponding reaction with alkenes.

the difference in rate can be attributed to the high-energy, vinylic carbocation intermediate that is formed when an alkyne is protonated.

The rate of alkyne hydration is markedly enhanced in the presence of mercuric sulfate (HgSO4), which catalyzes the reaction. Under these conditions, a mercuric ion (Hg2+) activates the alkyne toward nucleophilic attack, via the initial formation of a mercurinium ion.

Question 18

What is a ketone and what rxn generates it ?

Answer 18

acid -catalyzed Tautomerization produces a ketone

a ketone is an oxygen double bonded to c=o with two R groups coming off

Question 19

Tautomers

Answer 19

The enol and ketone are said to be tautomers, which are constitutional isomers that rapidly interconvert via the migration of a proton.

Question 20

Enol

Answer 20

A compound containing a hydroxyl group (OH) connected directly to a carbon-carbon double bond.

Question 21

keto-enol Tautomeriztion

Answer 21

The equilibrium that is established between an enol and a ketone in either acid-catalyzed or base-catalyzed conditions.

Generally, the ketone is highly favored, and the concentration of enol will be quite small.

Question 22

What are the reagents for base catalyzed reagents ?

Answer 22

1. Disiamylboarane ( R2BH or 9-BBN)
2. H2o2 NaOH

Question 23

what does a hydroboration oxidation of alkynes do

Answer 23

The reaction proceeds via an anti-Markovnikov addition, thereby installing a hydroxyl group at the less substituted position
The initial product of this reaction is an enol that cannot be isolated because it is rapidly converted into an aldehyde via tautomerization.

PRODUCES Aldehyde

Question 24

How to take a terminal alkyne to turn it into an aldehyde ( list the reagents in step order)

Answer 24

R -Tiple bond to aldehyde
1. R2BH
2. H2O2, NaOH

Question 25

aldehyde vs ketone

Answer 25

An aldehyde is an organic compound in which the carbonyl group is attached to a carbon atom at the END of a carbon chain R-(c=o)-H Ketone is an organic compound in which the carbonyl group is attached to a carbon atom within the carbon chain. R-(c=o)-R'

Question 26

Halogenation of Alkynes

Answer 26

halogenation. The one major difference is that alkynes have two π bonds rather than one and can, therefore, add two equivalents of the halogen to form a tetrahalide (Attach 4 halogens) reagents ( x = cl or Br) 1. X2 ( one equivalent) 2. CCl4 ^& this will reduce an alkyne to an alkene (double bond) both major (anti addition) and minor products (aka E and Z isomers ) 1. (excess) X2 2. CCl4 ^& this will reduce an alkyne to an alkane - tetrahalide ( 4 halogens attached) x x | | R - C - C - R | | x. x

Question 27

How does the product of Ozonolysis of Alkynes change when it is a terminal alkyne or not

Answer 27

When treated with ozone followed by water, alkynes undergo oxidative cleavage to produce carboxylic acids: ( carbon double bonded to O and single bond to R and OH) ( is it always a 1:2 ratio?) When a terminal alkyne undergoes oxidative cleavage, the terminal carbon atom is converted into carbon dioxide:

Question 27

Ozonolysis of Alkynes reagents

Answer 27

reagents 1. O3 2. H2O

Question 28

Alkylation of Terminal Alkynes

Answer 28

A reaction that achieves the installation of an alkyl group. For example, an SN2 reaction in which an alkyl group is connected to an attacking nucleophile. ?????? Alkylation reactions are very powerful since it can add alkyl groups to terminal alkynes by deprotonation with a strong base to produce the alkynide ion, which can then function as a nucleophile when treated with an alkyl halide. But this process is only efficient with methyl or primary alkyl halides, when secondary or tertiary alkyl halides are used the alkynide ion functions primarily as a base and elimination products are obtained.

Question 29

What are the two options for converting a triple bond to a double bond

Answer 29

H2, Lindlar's catalyst or Na, NH3(l)

Question 30

What are the options for converting a double bond back to a triple bond

Answer 30

1. Br, CCL4 2. xs NaNH2 3. H20

Question 31

The conjugate base of acetylene is called

Answer 31

acetylide ion ^ a relatively stabilized because the lone pair occupies an sp-hybridized orbital.

Question 32

The conjugate base of a terminal alkyne is called an

Answer 32

alkynide ion ^ which can only be formed with a sufficiently strong base, such as NaNH2.

Question 33

what are the two compounds that can be used to prepare a alkyne and how

Answer 33

Alkynes can be prepared from either geminal or vicinal dihalides via two successive E2 reactions.

Question 34

Catalytic hydrogenation of an alkyne yields

Answer 34

an alkane.

Question 35

Catalytic hydrogenation in the presence of a poisoned catalyst (Lindlar’s catalyst or Ni2B) yields...

Answer 35

a cis alkene.

Question 36

A dissolving metal reduction will convert an internal alkyne into a

Answer 36

trans alkene

Question 37

a dissolving metal reduction rxn involves

Answer 37

The reaction involves an intermediate radical anion and employs fishhook arrows, which indicate the movement of only one electron.

Question 38

Alkynes react with HX via what type of addition

Answer 38

Markovnikov addition

Question 39

what are the two possible mechanisms for hydrohalogentation of alkynes ?

Answer 39

One possible mechanism for the hydrohalogenation of alkynes involves a vinylic carbocation, while another possible mechanism is termolecular. note: Addition of HX to alkynes probably occurs through a variety of mechanistic pathways all of which are occurring at the same time and competing with each other.

Question 40

Treatment of a terminal alkyne with HBr and peroxides leads to

Answer 40

an anti-Markovnikov addition of HBr.

Question 41

Acid-catalyzed hydration of alkynes is catalyzed by mercuric sulfate (HgSO4) to produce an

Answer 41

enol that cannot be isolated because it is rapidly converted into a ketone.

Question 42

The interconversion between an enol and a ketone is called _________ and is catalyzed by ________?

Answer 42

keto-enol tautomerization and is catalyzed by trace amounts of acid or base.

Question 43

Hydroboration-oxidation of a terminal alkyne proceeds via

Answer 43

anti mark addition

Question 43

Hydroboration-oxidation of a terminal alkyne produces ______?

Answer 43

an enol that is rapidly converted into an aldehyde via tautomerization.

Question 44

In basic conditions, tautomerization proceeds via

Answer 44

a resonance-stabilized anion called an enolate ion

Question 45

tautomerization proceeds via a resonance-stabilized anion called an enolate ion occurs in what type of conditions

Answer 45

basic conditions

Question 46

Alkynes can undergo halogenation to form a

Answer 46

tetrahalide x x | | R - C - C - R | | x. x

Question 47

When treated with ozone followed by water, INTERNAL alkynes undergo oxidative cleavage to produce

Answer 47

carboxylic acids.

Question 48

When a terminal alkyne undergoes oxidative cleavage

Answer 48

the terminal side is converted into carbon dioxide.

Question 48

Alkynide ions undergo alkylation when treated with

Answer 48

an alkyl halide (methyl or primary)

Question 49

Acetylene possesses__________ and can undergo_______

Answer 49

two terminal protons and can undergo two separate alkylations.

Question 50

An alkene ( double bond) can be converted into an alkyne (triple bond) via

Answer 50

bromination followed by elimination with excess NaNH2 1. Br, CCL4 2. xs NaNH2 3. H20

Question 51

are terminal or internal alkynes more acidic

Answer 51

Terminal alkynes are acidic due to their high level of s character in the sp hybrid orbital and can be deprotonated with a suitable base.