Alkynes

Question 1

How is CaC2 prepared? How is ethyne prepared from that?

Answer 1

CaCO3 is decomposed at 2275K to form CaO which is then made to react with C
Add water leads to CaC2 + Ca(OH)2

Question 2

From dihalides how do u get alkynes? State the reagents used and why KOH isn’t the most ideal? Temperature?

Answer 2

heat it with alcoholic KOH and perform dehydrohalogenation.
2nd step is slow + yield is very less
alternative : stronger base - NaNH2 in liq NH3
192K

Question 3

Reagents from dehalogenation of tertrahalides?

Answer 3

Zn , CH3OH , heat

Question 4

State the GR of dehalogenation of haloforms.

Answer 4

2CHX3 + 6Ag ——–> HC—CH + 6AgX

just like wurtz reaction

Question 5

GR of Kolbe’s Electrolysis to form alkynes. K salts of which two acids?

Answer 5

COOK-CH2=CH2-COOK + H20 —-EC—–> CO2 + H2 + KOH + K2CO3 + HC—CH

Fumaric Acid & Maleic Acid

Question 6

GR of synthesizing higher alkynes from ethyne?

Answer 6

ethyne —-NaNH2 in liq NH3 197K—->

HC—C- Na (+NH3) + R-X —-> HC—CR + NaX

Question 7

Order of reactivity of alkanes, alkenes and alkynes?

Answer 7

Alkenes > Alkynes > Alkanes

Question 8

Why are alkenes more reactive than alkynes?

Answer 8

1. Alkynes are sp hybridized - highly electronegative due to 50% s character - holds electrons tightly - not available for reaction
2. Alkynes have cylindrical shape which delocalizes the pi electrons much more making it difficult to be available.

Question 9

Why are alkynes more reactive than alkanes?

Answer 9

Unsaturated + weak pi bonds

Question 10

ethyne + acid/base —->

Answer 10

salt + h2/h20

Question 11

How to get ethyne from a metal acetylide?

Answer 11

sodium acetylide + h20 —-> ethyne + naoh

Question 12

Formulas of the two heavy metal acetylides which react with ethyne? Colours of products?Byproducts? How to get back ethyne?

Answer 12

[Ag(NH3)2]+ OH- [Cu(NH3)2]+ OH-
white ppt , red ppt
NH3 and H2O
reaction with HCl or HNO3 to get ethyne and metal chloride or nitrate

Question 13

GR of alkyne with grignard reagent? Uses?

Answer 13

loses the acidic hydrogen and gets replaced with MgX and R-H forms.

1. separating terminal alkynes from alkanes, alkenes and alkynes
2. to differentiate between alkynes and terminal alkynes

Question 14

Why do alkynes have acidity?

Answer 14

triple bond - sp hybridized - 50% s character - pulls electrons towards itself - making it easy to abstract proton

Question 15

sp3 < sp2 < sp (acidity)
alkynes < h20
alkynes < carboxylic acids

Question 16

Addition of halogens to alkynes condition? Why ? what kind of alkene is formed and why? Comment on nature of halogens

Answer 16

CCl4 - overcomes the strong hold of the electrons
trans alkene formed to overcome steric hinderance
cl, br - easily I - huge so only trans alkene

Question 17

Important catalyst for the addition of hydrogen halide to alkyne?

Answer 17

Hg2+ or Ba2+

Question 18

Addition of HCl to ethyne, why only 1 step?

Answer 18

second step does occur but dehydrohalogenation occurs and goes back to vinyl chloride.

Question 19

Addition of hypohalous acid to alkynes - catalyst? what does it form and convert to?

Answer 19

Hg2+

forms gem diol - dehydration (isomerization) to aldehydes or ketones

Question 20

Addition of water to alkynes - reaction conditions? What does it form?

Answer 20

Acid (H+) , Hg2+, heat of 333K

Enol —-> tautomerizes to ketone or aldehyde

Question 21

Reaction conditions for addition of carboxylic acid to ethyne? Uses of step 1 and step 2 products?

Answer 21

Hg2+ and 353K
1 - vinyl acetate - to make vinyl resin
2 - ethylidene diacetate - acetic anhydride

Question 22

Reaction condition of addition of HCN to ethyne? Use?

Answer 22

Ba(CN)2

vinyl cyanide or acrylonitrile is used as a starting material for polymers

Question 23

Addition of a nucleophile - CH3OH to ethyne : conditions? How is the catalyst formed? Product?

Answer 23

koch3 : K+ OCH3- 433-473K
ROH + K —-> K+OR-
H2C=CH-OCH3

Question 24

Reduction of alkynes - catalytic reduction : change of catalysts gives different alkenes.

Answer 24

In birch reduction, trans is formed as trans free radical is more stable than cis free radical

Question 25

Catalytic Oxidation of Alkynes : To Dicarbonyl Compounds (carboxylic acids)

Answer 25

cold dil KMnO4 (298-303K)
2 pi bonds break and O double gets added and O between H
if ketone and acid together gets further oxidized to give acid + CO2

Question 26

Catalytic Oxidation of Alkynes : To Carboxylic Acids

Answer 26

Hot KMnO4 (373-383K)
all bonds break, 2 fragments of acids
O double is added and Oh is added  ( h remaining same)

Question 27

Catalytic Oxidation of Alkynes : To Dicarbonyl Compounds (aldehydes and ketones)

Answer 27

Ozonolysis
O3 in CCl4 w 196K along with Zn dust in water
only 2 bonds break - ozonide and then double O

Question 28

Linear polymerization : ethyne becomes? Conditions?

Answer 28

HT, HP , catalyst

Question 29

Cyclic polymerization conditions? How many ethnes to form benzene and octa-1,3,5,7-tetrayne?

Answer 29

Red hot Fe tube at 873K

3 and 4

Question 30

Isomerization : shifting triple bonds - conditions?

Answer 30

NaNH2 in inert solvent like CCl4

Question 31

why are alkanes, alkenes and alkynes insoluble in water?

Answer 31

As C increase - the non polar groups goes on increasing

Question 32

what happens to the boiling points? in isomers?

Answer 32

As C increases - surface area increases - interaction increases - waander vaal forces of interaction is more - more energy

branched < straight chain (as branches has lesser surface area)

Question 33

In alkanes regarding MP what is seen? Why?

Answer 33

Alternation effect
even C > odd C
due to even - symmetry - tightly packed - more energy