Alkenes and Alkynes - URI Department of Chemistry · Calculating a Degree of Unsaturation 7.2...
Transcript of Alkenes and Alkynes - URI Department of Chemistry · Calculating a Degree of Unsaturation 7.2...
3/8/2011
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John E. McMurry
http://www.cengage.com/chemistry/mcmurry
Richard Morrison • University of Georgia, Athens
Chapter 7
Alkenes and Alkynes
Problems: 1-21, 25-28, 31-35, 38-56
Alkenes and Alkynes
Alkene (or “olefin”)
• Hydrocarbon that contains a carbon-carbon double bond
• Present in most organic and biological molecules
• β-Carotene
Alkyne
• Hydrocarbon that contains a carbon-carbon triple bond
• Rarely occur in biological molecules or pathways
Alkenes and Alkynes
Ethylene and propylene
are the two most
important industrially
produced organic
chemicals
• Produced by
“cracking” C2-C8
alkanes upon heating
to temperatures up to
900 oC.
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Because of its double bond an alkene has fewer hydrogens than an alkane with the same number of carbons and is therefore referred to as unsaturated
Alkene Alkane
CnH2n CnH2n+2
C2H4 C2H6
7.1 Calculating a Degree of Unsaturation
Degree of unsaturation
• Number of rings and/or multiple bonds present in the
molecule
• Unknown hydrocarbon of formula C6H10with molecular weight
of 82 has two fewer pairs of hydrogens (H14 – H10 = H4 = 2H2)
than a saturated hydrocarbon
• Degree of unsaturation is two
• Possible structures for unknown
Calculating a Degree of Unsaturation
Calculations for compounds containing other elements in addition to carbon and hydrogen:
• Organohalogen compounds (C,H,X, where X = F, Cl, Br, or I)
• Add number of halogens and hydrogens to arrive at an equivalent hydrocarbon formula
Calculating a Degree of Unsaturation
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• Organooxygen compounds (C,H,O)
• Oxygen does not affect the formula of an equivalent
hydrocarbon
• Ignore the number of oxygens
Calculating a Degree of Unsaturation
• Organonitrogen compounds (C,H,N)
• Has one more hydrogen than a related hydrocarbon
• Subtract the number of nitrogens from the number of
hydrogens for equivalent hydrocarbon formula
Calculating a Degree of Unsaturation
7.2 Naming Alkenes and Alkynes
• Naming Alkenes
• Similar to the naming rules for alkanes
• Step 1 - Name the parent hydrocarbon
• Find longest carbon chain containing the double bond
• Step 2 – Number the carbon atoms in the chain
• Double-bond carbons should receive lowest possible numbers
• Begin at end nearer first branch point if double bond is equidistant
from the two ends
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• Step 3 - Write the full name using suffix -ene in place of -
ane
• If more than one double bond indicate position and use
suffixes -diene, -triene, and so on
Naming Alkenes and Alkynes
• Older naming system still in use which places the locant, or
number locating the position of the double bond, at the
beginning of the name
Naming Alkenes and Alkynes
• Cycloalkenes
• Number cycloalkene so double bond is between C1
and C2 and first substituent has lowest number
possible
Naming Alkenes and Alkynes
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Common names that are often used and are recognized
by IUPAC:
Naming Alkenes and Alkynes
Alkynes
• Named just like alkenes using suffix -yne
• Number main chain so triple bond receives as low a
number as possible
Alkyl, alkenyl, and alkynyl groups:
Naming Alkenes and Alkynes
7.3 Cis-Trans Isomerism in Alkenes
• Carbon-carbon double bond description
• Valence bond language
• Carbons are sp2 hybridized
• Three equivalent hybrid orbitals that lie in a plane at angles of
120º to one another
• Carbons form a σ bond by head-on overlap of sp2 orbitals and a p
bond by sideways overlap of unhybridized p orbitals oriented
perpendicular to sp2 plane
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• Molecular orbital language
• Interaction between p orbitals leads to one bonding and one antibonding molecular orbital
• bonding MO contains no node between nuclei
• antibonding MO contains a node between nuclei resulting from combination of lobes with different algebraic signs
Cis-Trans Isomerism in Alkenes
Free rotation is not possible around a double bond
• The barrier to double bond rotation must be at least as great
as the strength of the bond itself (~ 350 kJ/mol)
Cis-Trans Isomerism in Alkenes
Disubstituted alkene• Two substituents other than hydrogen are attached to the double-bond
carbons
• But-2-ene
• Two isomers cannot interconvert spontaneously
• Methyl groups are cis- or trans- to each other
• Bond rotation cannot occur – the two but-2-enes are cis-trans stereoisomers
Cis-Trans Isomerism in Alkenes