Download Alkynes: Properties, Naming, Preparation, and Reactions - Prof. Peter B. Balanda and more Study notes Organic Chemistry in PDF only on Docsity! Alkynes 2 CH4 >1150 o C HC CH + 3 H2 steam H2C CH2 >1150 o C HC CH + H2 steam Petroleum ―cracking‖ Ethyne (Acetylene), the smallest alkyne Naming Alkynes • General hydrocarbon rules apply with ―-yne‖ as a suffix indicating an alkyne • Numbering of chain with triple bond is set so that the smallest number possible include the triple bond Diynes, Enynes, and Triynes • A compound with two triple bonds is a diyne – An enyne has a double bond and triple bond – A triyne has three triple bonds • Number from chain end nearest a double or triple bond – double bonds have priority if both are present in the same relative position Alkynes as substituents are called “alkynyl”
CH,CH,CH,CH, > CH,CH,CH= CH CH,CH,C=c+
Butyl 1-Butenyl 1-Butynyl
(an alkyl group) (a vinylic group) (an alkynyl group)
©2004 Thomson - Brooks/Cole
Br H OH OH Preparation of Alkynes by Dehydrohalogenation Alkyne forms through a vinylic halide intermediate The ability of vinylic halides to undergo elimination has been demonstrated Why 2 equivalents of the strong base NaNH2? Ans. The 1st eq. removes the hydroxyl proton Reactions of Alkynes: • Addition reactions of alkynes are similar to those of alkenes • Intermediate alkene reacts further with excess reagent • Regiospecificity according to Markovnikov Hydrohalogenation and Halogenation • Addition of HX and X2 usually gives Anti Addition (Br and Br trans to each other) Addition of HX to Alkynes Probably Involves Vinylic Carbocations
C=C +C—C—H C—C
/\ JT SH
H H H H H H
An alkene An alkyl carbocation An alkyl bromide
fou) Br H
H fy + / :Bri- \ /
RC=CH R—C=C ——_—_ c=C
\ /' x
H R H
An alkyne A vinylic carbocation A vinylic bromide
©2004 Thomson - Brooks/Cole
Addition of HX to Alkynes Involves Vinylic Carbocations • Addition of H-X to alkyne should produce a vinylic carbocation intermediate – Secondary vinyl carbocations form less readily than primary alkyl carbocations – Primary vinyl carbocations probably do not form at all Mercury(II)-Catalyzed Hydration of Alkynes • Alkynes do not react with aqueous protic acids • Mercuric ion (as the sulfate) is a Lewis acid catalyst that promotes addition of water in Markovnikov orientation • The immediate product is a vinylic alcohol, or enol, which spontaneously transforms to a ketone Keto-enol Tautomerism • Isomeric compounds that can rapidily interconvert by the movement of a proton are called tautomers and the phenomenon is called tautomerism • Enols rearrange to the isomeric ketone by the rapid transfer of a proton from the hydroxyl to the alkene carbon • The keto form is usually so stable compared to the enol that only the keto form can be observed The alkyne uses a pair of electrons to
attack the electrophilic mercury(I)
ion, yielding a mercury-containing
vinylic carbocation intermediate.
Nucleophilic attack of water on
the carbocation forms a C—O bond
and yields a protonated mercury-
containing enol.
Abstraction of H* from the
protonated enol by water gives
an organomercury compound.
Replacement of Hg* by H* occurs
to give a neutral enol.
The enol undergoes tautomerization
to give the final ketone product.
©2004 Thomson - Brooks/Cole
Problem 8.5 What alkynes would you start with to prepare the following ketones? CH3CH2CH2C CH CH3CH2C CCH3 Hydroboration/Oxidation of Alkynes • BH3 (borane) adds to alkynes to give a vinylic borane • Oxidation with H2O2 produces an enol that converts to the ketone or aldehyde • Process converts alkyne to ketone or aldehyde with orientation opposite to mercuric ion catalyzed hydration Problem 8.6 What alkyne would you start with to prepare each of the following compounds by a hydroboration/oxidation reaction? C CH C C CHCH3 CH3 CH3CH CH3 Conversion of Alkynes to cis-Alkenes • Addition of H2 using chemically deactivated [(Pb(OAc)2] palladium on calcium carbonate as a catalyst (the Lindlar catalyst) produces a cis alkene • The two hydrogens add syn (from the same side of the triple bond) Conversion of Alkynes to trans-Alkenes • Anhydrous ammonia (NH3) is a liquid below -33 ºC – Alkali metals dissolve in liquid ammonia and function as reducing agents • Alkynes are reduced to trans alkenes with sodium or lithium in liquid ammonia • The reaction involves a radical anion intermediate (see Figure 8-4, next slide) Lithium metal donates an electron to
the alkyne to give an anion radical...
... Which abstracts a proton from
ammonia solvent to yield a vinylic
radical.
The vinylic radical accepts another
electron from a second lithium atom
to produce a vinylic anion ...
... which abstracts another proton
from ammonia solvent to yield the
final trans alkene product.
©2004 Thomson - Brooks/Cole
_f\ Ns _
R—C=C—H +:NH, Nat —~> R—C=C:- Nat + :NH;
Acetylide anion
©2004 Thomson - Brooks/Cole
Alkyne Acidity: Formation of Acetylide Anions • Terminal alkynes are weak Brønsted acids (alkenes and alkanes are much less acidic (pKa ~ 25). See Table 8.1, next slide, for comparisons) • Reaction of strong anhydrous bases with a terminal acetylene produces an acetylide ion • The sp-hydbridization at carbon holds negative charge relatively close to the positive nucleus (see figure 8-5)
TABLE 8.1 Acidity of Simple Hydrocarbons
Type Example K, pK,
Alkyne HC=CH 1O=+ 25 Stronger acid
Alkene H,C=CHy 10-44 44
Alkane CH, = 1OEe 60 Weaker acid
©2004 Thomson - Brooks/Cole
The nucleophilic acetylide anion uses
its electron lone pair to form a bond
to the positively polarized, electro-
philic carbon atom of bromomethane.
As the new C-C bond begins to form,
the C—Br bond begins to break in the
transition state.
The new C-C bond is fully formed
and the old C—Br bond is fully
broken at the end of the reaction.
This is called an
Sy2 mechanism
© 2004 Thomson/Brooks Cole
H
‘|
ne
H—C=C:--G:-Br + Nat
\
HH
Transition state
|
H
/
H—c=c—c.
“H
H
+ NaBr
Alkylation of Acetylide Anions • Reaction with a primary alkyl halide produces a hydrocarbon that contains carbons from both partners, providing a general route to larger alkynes Limitations of Alkyation of Acetylide Ions • Reactions only are efficient with 1º alkyl bromides and alkyl iodides • Acetylide anions can behave as bases as well as nucelophiles • Reactions with 2º and 3º alkyl halides gives dehydrohalogenation, converting alkyl halide to alkene