Ch09 - Alkynes Alkynes or acetylenes are compounds that...

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Alkynes Alkynes or acetylenes are compounds that contain a carbon–carbon triple bond. E.g. HCC acetylene CH 3 CH 2 CCH H 3 CCCC H 3 H The triple bond results in a molecular formula of C n H 2n-2 Ethane C 2 H 6 0 elements of unsaturation Ethene C 2 H 4 1 element of unsaturation Ethyne C 2 H 2 2 elements of unsaturation The triple bond contributes two elements of unsaturation. Nomenclature of Alkynes IUPAC nomenclature is similar to that for alkenes, except the –ane ending is replaced with –yne. The chain is numbered from the end closest to the triple bond. ethyne H 3 H 3 H 2 H 2-butyne H H 3 C CH 3 C Br H CH 3 6-bromo-2-methyl-3-heptyne When additional functional groups are present, the suffixes are combined. H 2 C H 3 H 3 C H CCCH H 3 CCC H C CH 2 CH 3 OH OCH 3 CH 3 2-methyl-1-penten-3-yne 3-butyn-2-ol 4-methoxy-2-hexyne Ch09 Alkynes Page1
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Terminal and Internal Alkynes The position of the triple bond can alter the reactivity of the alkyne. Compounds with triple bonds at the end of a molecule are called terminal alkynes. (Terminal C-H groups are called acetylenic hydrogens). H 3 CCCC H 3 H 3 CCCH terminal alkyne internal alkyne If the triple bond is flanked by alkyl groups on both sides it is an internal alkyne. Electronic Structure of Alkynes The triple bond consists of one σ bond and two π bonds. Each carbon is bonded to two other atoms, and there are no nonbonding electrons. Carbon requires two hybrid orbitals to bond to the atoms, thus sp hybrids are used. The sp orbitals are linear and oriented at 180°. The C-C bond is formed from sp-sp overlap The C-H bond is formed from sp-s overlap. Ch09 Alkynes Page2
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The creation of sp hybrids leaves 2 free p orbitals – these create the two π bonds. The p orbitals overlap just as for an alkene, except there are two π bonds at right angles to one another. This generates a cylinder of electron density around the sigma bond. The C-C bond length for ethyne is 1.20Å which is shorter than ethane (1.54Å) and ethene (1.33Å). The C-H bond length in ethyne is 1.06Å which is also shorter than in ethane (1.09Å) or ethene (1.08Å). This is because the C-H bond contains more s character (sp 3 sp 2 sp) which gives stronger bonds. Ch09 Alkynes Page3
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Acidity of Alkynes Terminal alkynes are acidic , the end hydrogen can be removed as a proton by a strong base.
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Ch09 - Alkynes Alkynes or acetylenes are compounds that...

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