Alkynes Flashcards
How is CaC2 prepared? How is ethyne prepared from that?
CaCO3 is decomposed at 2275K to form CaO which is then made to react with C
Add water leads to CaC2 + Ca(OH)2
From dihalides how do u get alkynes? State the reagents used and why KOH isn’t the most ideal? Temperature?
heat it with alcoholic KOH and perform dehydrohalogenation.
2nd step is slow + yield is very less
alternative : stronger base - NaNH2 in liq NH3
192K
Reagents from dehalogenation of tertrahalides?
Zn , CH3OH , heat
State the GR of dehalogenation of haloforms.
2CHX3 + 6Ag ——–> HC—CH + 6AgX
just like wurtz reaction
GR of Kolbe’s Electrolysis to form alkynes. K salts of which two acids?
COOK-CH2=CH2-COOK + H20 —-EC—–> CO2 + H2 + KOH + K2CO3 + HC—CH
Fumaric Acid & Maleic Acid
GR of synthesizing higher alkynes from ethyne?
ethyne —-NaNH2 in liq NH3 197K—->
HC—C- Na (+NH3) + R-X —-> HC—CR + NaX
Order of reactivity of alkanes, alkenes and alkynes?
Alkenes > Alkynes > Alkanes
Why are alkenes more reactive than alkynes?
- Alkynes are sp hybridized - highly electronegative due to 50% s character - holds electrons tightly - not available for reaction
- Alkynes have cylindrical shape which delocalizes the pi electrons much more making it difficult to be available.
Why are alkynes more reactive than alkanes?
Unsaturated + weak pi bonds
ethyne + acid/base —->
salt + h2/h20
How to get ethyne from a metal acetylide?
sodium acetylide + h20 —-> ethyne + naoh
Formulas of the two heavy metal acetylides which react with ethyne? Colours of products?Byproducts? How to get back ethyne?
[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
GR of alkyne with grignard reagent? Uses?
loses the acidic hydrogen and gets replaced with MgX and R-H forms.
- separating terminal alkynes from alkanes, alkenes and alkynes
- to differentiate between alkynes and terminal alkynes
Why do alkynes have acidity?
triple bond - sp hybridized - 50% s character - pulls electrons towards itself - making it easy to abstract proton
sp3 < sp2 < sp (acidity)
alkynes < h20
alkynes < carboxylic acids
Addition of halogens to alkynes condition? Why ? what kind of alkene is formed and why? Comment on nature of halogens
CCl4 - overcomes the strong hold of the electrons
trans alkene formed to overcome steric hinderance
cl, br - easily I - huge so only trans alkene
Important catalyst for the addition of hydrogen halide to alkyne?
Hg2+ or Ba2+
Addition of HCl to ethyne, why only 1 step?
second step does occur but dehydrohalogenation occurs and goes back to vinyl chloride.
Addition of hypohalous acid to alkynes - catalyst? what does it form and convert to?
Hg2+
forms gem diol - dehydration (isomerization) to aldehydes or ketones
Addition of water to alkynes - reaction conditions? What does it form?
Acid (H+) , Hg2+, heat of 333K
Enol —-> tautomerizes to ketone or aldehyde
Reaction conditions for addition of carboxylic acid to ethyne? Uses of step 1 and step 2 products?
Hg2+ and 353K
1 - vinyl acetate - to make vinyl resin
2 - ethylidene diacetate - acetic anhydride
Reaction condition of addition of HCN to ethyne? Use?
Ba(CN)2
vinyl cyanide or acrylonitrile is used as a starting material for polymers
Addition of a nucleophile - CH3OH to ethyne : conditions? How is the catalyst formed? Product?
koch3 : K+ OCH3- 433-473K
ROH + K —-> K+OR-
H2C=CH-OCH3
Reduction of alkynes - catalytic reduction : change of catalysts gives different alkenes.
In birch reduction, trans is formed as trans free radical is more stable than cis free radical
