What is an alkyne?
Hydrocarbons that contain C≡C bond(s)
What is the basic structure of an alkyne?
The C≡C bond is composed of one sigma bond, and 2 pi bonds
These carbons have sp hybridisation
Each carbon has 2sp orbitals(form sigma bond) and 2 2p orbitals(forms 2 pi bonds)
Has linear geometry
Can go to a single bond by donating electrons
Alkyne nomenclature
Use -yne prefix e.g. propyne
1. The longest chain containing the C≡C bond is the root name
2. C≡C bond must have lowest number possible
3. First carbon number indicates location
4. General rules for multiple C≡C bonds
5. Indicate both C≡C bonds and C=C bonds
Alkyne acidity trends
Acidity increases as follows: sp3<sp2<sp
Alkynes are the most acidic hydrocarbon
Only applies when a ≡ carbon is attached to a hydrogen
How are alkynes synthesized?
Using acetylide ions
Elimination reactions
How are acetylide ions formed, and used to create alkynes?
A terminal alkyne reacts with a strong base to form this ion
It then reacts with an alkyl halide via SN2 to form a new alkyne
What is a vicinal dihalide?
What is a geminal dihalide?
Halogens on adjacent carbon atoms
Halogens on the same carbon atom
How are elimination reactions used to create alkynes?
Via dehydrohalogenation of vicinal or geminal dihalides in the presence of a strong base
Steps:
1. The dihalide is converted to a haloalkene (base attacks hydrogen, which then attacks one of the halogens)
2. The haloalkene is converted to an alkyne (Base attacks other hydrogen, which attacks the remaining halogen, electrons form pi bond)
Which reactions do alkynes undergo?
Addition reactions:
-Catalytic hydrogenation
-Metal-ammonia reduction
-Halogen addition
-Hydrogen halide addition
-Hydration reactions
-Hydroboration
Explain catalytic hydrogenation of alkynes
Uses catalysts like: Ni, Pt, Pd
Products depend on amount of Hydrogen molecules (1 mole: alkyne to alkene; 2 moles: alkyne to alkane)
Explain metal-ammonia reduction of alkynes
Involve reagents such as Li or NH3
2 Hydrogens added to C≡C bond on opposite sides
Forms trans alkenes
Explain halogen addition to alkynes
A halogen atom adds to each carbon of the C≡C bond
syn-and-anti manners, often produces cis and trans dihaloalkenes
Equimolar conditions = dihaloalkenes
Excess halogens = tetrahalides
Explain addition of hydrogen halides to alkynes
Uses carbocation intermediate (Same rules as hydrohalogenation of alkenes)
Results in alkenyl halides in equimolar conditions
Results in geminal dihalides if H-X is in excess
In the presence of peroxides, follows anti-Markovnikov rule
Explain hydration of alkynes to ketones or aldehydes
An OH group attaches to the C≡C bond.
This causes an enol, which converts to a ketone or an aldehyde
(Called keto-enol tautomerism)
Can be acid, or mercuric ion catalysed
Explain hydroboration of alkynes
Adding water to an alkyne in the presense of BH3(strong lewis acid)
H and OH add to the C≡C bond via the anti-Markovnikov rule
Produces an enol which converts to an aldehyde
