Chapter 8: Alkynes Naming Alkynes Structure Common Name IUPAC HCHC Acetylene ethyne HCCHC 3 methyl acetylene propyne H3C CCCH3 dimethyl acetylene 2-butyne HCCHC 2 CH3 ethyl acetylene 1-butyne HC=C- as substituent is called ethynyl.
Name these: H3C H3C CH3
CH CH
H3C H3C CH3
Some things to consider: The closest unsaturation to the end takes precedence:
CH3
H3C CH H3C
(4Z)-4-hexen-1-yne
Double bond takes precedence if equal
H3C
CH3
(2E)-2-hexen-4-yne 1-buten-3-yne
Reactivity of alkynes Addition reactions: Br fast slow - + Br C CH RRC C HBr RRC CH vinyl cation R R The addition of HBr to an alkyne is slower than an alkene because of two factors: • the sp π orbital is less basic than the sp2 π orbital. Increased s character. • the vinyl cation formed is unstable compared to an alkyl cation. Here the sp hybridized C+ cannot be stabilized by hyperconjugation because of the pi bond perpendicular to it.
+ H2C C H
hybrid? Stability of vinyl C+ is about the same as H3C+.
Comparison of reactivity of alkenes vs alkynes with HBr: Alkene Alkyne EA1 EA2 ∆H
http://www.cem.msu.edu/~reusch/VirtualText/addyne1.htm#add1
General Addition of HBr Br fast slow - + Br C CH H3C C C CH3 HBr H3C C CH CH3 1st mole vinyl cation H3C CH3 + Br Br Br fast + C CH HBr C CH2 C CH2 - Br H C CH 2nd mole 3 3 H3C CH3 H3C CH3
Example:
http://www.chemistry.sci.kun.nl/onderwijs/oc1b/2002/College%20H09.pdf General Addition of Br2 Br CH3 slow C C H3C C C CH3 Br Br trans from anti addition 1st mole H3C Br Br CH + 3 slow Br - Br C C Br Br Br CH3 2nd mole H3C Br H3C Br
Examples:
http://www.chemistry.sci.kun.nl/onderwijs/oc1b/2002/College%20H09.pdf Addition of Water and interconversion of the (enol) vinyl alchol to a ketone.
H + HO HO H2O HgSO4 + + C CH + 2 H2O C CH2 C CH H3O H3C C CH HOH2 2 H3C H C 3 H3C enol What is the role of acid in this reaction? How many moles of water are consumed? The enol is unstable compared to a unique rearrangement called tautonomerization. The enol continues to react in the presence of acid.
+ H H O O HO O H O + 2 H O + 3 + C CH3 C CH2 HOH2 C CH 3 H C CH H C 3 3 H C 3 3 H3C This process is called keto-enol tautonomerization. What is the role of acid in this reaction? How many moles of water are consumed?
What is the definition of a tautonomer?
CH2 O H C H H C O 3 H3C CH2 enol ketone The substitution pattern for alkynes follows Markovinikov’s rule:
HC≡CH + H2O + HgSO4 & H2SO4 ——> [ H2C=CHOH ] ——> H3C-CH=O
RC≡CH + H2O + HgSO4 & H2SO4 ——> [ RC(OH)=CH2 ] ——> RC(=O)CH3
RC≡CR' + H2O + HgSO4 & H2SO4 ——>
[ RHC=C(OH)R' + RC(OH)=CHR' ] ——> RCH2-C(=O)R' + RC(=O)-CH2R' http://www.cem.msu.edu/~reusch/VirtualText/addyne1.htm#add1 Examples:
http://www.chemistry.sci.kun.nl/onderwijs/oc1b/2002/College%20H09.pdf
H OH O keto-enol tautonomerism
CH2 R R RR
Hydroboration
H H BH BH2 2 further alkylations R CH + BH3 R R H H note E orientation
H O H OH O H BR3 2 2 keto-enol tautonomerism
CH2 OH- (aq) R H R H RH
The complete reaction is written like this. O HgSO4 R CH dilute H2SO4 R CH3
O 1. BH3-THF R CH 2. H O , aq base 2 2 RCH2 H
Usually, BH3-THF makes aldehydes and Hg catalyzed hydration makes ketones.
Reduction H2 reduces alkynes to alkenes and then to alkanes. The fact that the reaction is exothermic and moreso for alkynes than alkenes, the reaction is hard to stop.
The reaction takes place on the surface of Pt or Pd. Since the addition is twice, it is hard to see the syn addition.
Reduction of alkynes to (Z) - alkenes Lindlar’s catalyst: Pd, quinoline, Pb and CaCO3 poisons the metal catalyst, so that the H2 adds only to the alkyne – not reactive enough for alkenes.
Reduction of alkynes to (E) – alkenes. Reaction via Na radical dissolved in liquid NH3.
Mechanism of Na/NH3 reductions: + Na Na NH (e-) R H -33C + 3
R R H NH2 NH - R R + NH3 (e-) 2 -33C R -33C R R H R
H R H R H R + NH3 (e-) H NH2 NH - -33C 2 R R R -33C R H
H
Oxidation Like alkenes, either with O3 (ozone) or KMnO4. Because of the added unsaturation of alkynes, it is hard to stop at an aldehyde. So, the reaction goes mostly to carboxylic acids. Terminal alkynes will make fizzy water.
O O KMnO4 + CO2 + H2O H3C CH H3C OH HOHO
O O 1. O3 + H C R 3 H C OH HRO 2. H2O2 (aq) 3
Acid/Base What is a suitable base to make the reaction go to the right below.
R CH + B:- R C: + BH
For pKa of BH NH3 33 HC≡CH 25 H2O 16 + NH4 9
Alkylation Acetylide anion RC≡C:- can react as a nucleophile and react with a 10 alkyl bromide in a substitution reaction to make a longer alkyne. This process of adding an alkyl group and is called alkylation. NaNH2 Step 1 H C CH 3 H3C C:
H C C: Step 2 3 + H3C Br H3C CH3
MultiStep synthesis • Prepare cis-4-octene from 1-pentyne Step 1: 1-pentyne to pentynyl anion
Step 2: 1-pentynyl anion to 4-octyne
Step 3: 4-octyne to (Z)-4-octene
• Prepare trans-2-hexene from 1-pentyne