Chap-9A - Chapter 9 Alkynes 9-1 Introduction Alkynes...

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Chapter 9 Alkynes
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Chapter 9 2 9-1 Introduction • Alkynes contain a triple bond. • General formula is C n H 2n-2 • Two elements of unsaturation for each triple bond. • Some reactions are like alkenes: addition and oxidation. • Some reactions are specific to alkynes.
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Chapter 9 3 9-2 Nomenclature of Alkynes (IUPAC) • Find the longest chain containing the triple bond. • Change -ane ending to -yne . • Number the chain, starting at the end closest to the triple bond. • Give branches or other substituents a number to locate their position.
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Chapter 9 4 E.g.: Name these: CH 3 CH CH 3 CH 2 C C CH CH 3 CH 3 CH 3 C C CH 2 CH 2 Br CH 3 C CH propyne 5-bromo-2-pentyne 2,6-dimethyl-3-heptyne
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Chapter 9 5 Additional Functional Groups • All other functional groups, except ethers and halides have a higher priority than alkynes. • For a complete list of naming priorities, look inside the back cover of your text.
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Chapter 9 6 Examples CH 2 CH CH 2 CH CH 3 C CH 4-methyl-1-hexen-5-yne CH 3 C C CH 2 CH OH CH 3 4-hexyn-2-ol See p.383 for more details.
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Chapter 9 7 Common Names Named as substituted acetylene. CH 3 C CH methylacetylene CH 3 CH CH 3 CH 2 C C CH CH 3 CH 3 isobutylisopropylacetylene
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Chapter 9 8 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). • If the triple bond is flanked by alkyl groups on both sides it is an internal alkyne. H 3 C C C C H 3 H 3 C C C H terminal alkyne internal alkyne CH 3
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Chapter 9 9 9-3 Physical Properties of Alkynes • Nonpolar, insoluble in water. • Soluble in most organic solvents. • Boiling points similar to alkane of same size. • Less dense than water. • Up to 4 carbons, gas at room temperature.
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Chapter 9 10 9-5 Electronic Structure of Alkynes • The sigma bond is sp-sp overlap. • The two π bonds are unhybridized p overlaps at 90 ° , which blend into a cylindrical shape.
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11 Bond Lengths • More s character, so shorter length. • Three bonding overlaps, so shorter.
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Chap-9A - Chapter 9 Alkynes 9-1 Introduction Alkynes...

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