Alkynes I.Nomenclature and Properties A.Naming Alkynes 1)Carbon-Carbon triple bond, C n H 2n-2 2)IUPAC a)Replace the –ene of an alkene with –yne b)Give the number where the triple bond occurs 3)Common Names: derivatives of acetylene 4)Terminal alkyne has one unsubstituted C, Internal Alkyne does not 5)As a substituent, an alkyne is named alkynyl Ethyne (Acetylene) 2-Butyne Dimethylacetylene 4-bromo-2-hexyne3,3-dimethyl-1-butyne Ethynyl 2-propynyl

6)Other examples: 7)Molecule with double and triple bond is an alkenyne, C=C lowest # 8)Molecule with triple bond and hydroxyl group is an alkynol B.Properties of Alkynes 1)Bonding a)Carbon-Carbon bond is made of on  (sp-sp) and 2  bonds b)C—H  -bonds made of Csp and H1s overlap trans-1,2-diethynylcyclohexane 2-propynylcyclopropane 2-propyn-1-ol 3-hexen-1-yne 1-penten-4-yne 5-hexyn-2-ol

c)C-C triple bond DH = 229 kcal/mol (> double or single bond) d)C-H bond DH is 131 kcal/mol, stronger than alkene or alkane e)Weak  -bonds are the most reactive area f)C-C Bond length is shorter than alkene or alkane, as is C-H bond g)Linear geometry is result of sp hybridization

2)Boiling points are similar to alkenes and alkanes (fairly nonpolar) 3)Acidity a)Terminal alkynes are much more acidic than alkanes or alkenes b)sp hybridization makes C more electronegative than usual c)Ethyne pK a = 25, ethene = 44, ethane = 50 d)Bases used to deprotonate alkynes: e)Resulting anion is called an Alkynyl Anion is a good nucleophile C.Spectroscopy 1)Proton NMR a)Alkynyl H are shielded by  -electron local magnetic field b)This is the opposite effect as seen in alkenes c)Geometry dictates the effect of the local field

d)Local field opposes the NMR field at the H locations: shielded e)  = for alkyne hydrogens f)Triple bond does transmit spin-spin coupling well. J = 2-4 Hz Long Range 4-bond coupling

2) 13 C NMR a)Alkyne Carbons  = ppm (alkenes = , alkanes = 5-45) b)Examples: 3)IR a)C≡C—H = cm -1 b)C≡C = cm -1 (weak for internal alkynes)

II.Stability of the Triple Bond A.Alkynes are high energy compounds Acetylene (ethyne) torches produce much heat for welding or cutting B.Internal Alkynes are most stable 1)Heats of Hydrogenation to alkane a)1-butyne = kcal/mol b)2-butyne = kcal/mol 2)Hyperconjugation makes internal more stable III.Preparations of Alkynes A.Double Elimination 1)Alkenes can be prepared by E2 reactions with alkyl halides

2)Alkynes can be prepared from a vicinal dihaloalkane base 3)Details: a.3 eq. Base needed because alkyne will deprotonate when formed b.H 2 O re-protonates the alkyne c.NH 3 (l) boils away when reaction is done (b.p. = -33 o C) d.Stereochemistry of the Intermediate doesn’t matter “alkenyl halide”

2)Halogenation-Double Dehydrohalogenation Alkene to Vicinal Dihaloalkane to Alkyne B.Alkylation of Alkynyl Anions 1)Terminal Alkynes can be made into internal alkynes 2)Alkyl and Alkenyl Metal reagents don’t attack haloalkanes fast enough 3)Alkynyl Anion can react with haloalkanes

5)Secondary and Tertiary Haloalkanes give E2 products instead of S N 2 a.Alkynyl anion is a strong base b.S N 2 Reaction works much better with primary haloalkanes 6)Ethyne can be mono- or disubstituted 7)Alkynyl Anions can do other Nu reactions as well