Topic 5D Alkenes and alkynes

5D Alkenes and alkynes A sigma (  ) bond between two sp 2 hybridised carbons A pi (  ) bond formed by sideways overlap of two unhybridized p z orbitals 42 Structure of double bond :

Pi bonding Pi  bonds weaker than sigma bonds Alkenes are more reactive than alkanes. Electrons in p-orbitals further from the nucleus than s- electrons and are less tightly bound Reactivity of alkenes involves the  -bond, not the  -bonds. 43 Bond energy: Total — Sigma = Pi kJmol –1

Structural consequences of  -bonding 43 Rotation means loss of  overlap Movie from Saunders General Chemistry CD-ROM

Structural consequences of  -bonding Whereas in alkanes there is free rotation about C—C bonds, Alkenes are rigid For rotation the  -bond must be broken requiring 285 kJmol –1 43

Rotation in C—C bonds Staggered and eclipsed conformations possible Eclipsed higher in energy by 11 kJmol –1 43

Restricted rotation in C=C bonds Alkenes are however rigid and can have two configurations Groups on the double bond may be cis or trans eg 2-butene 44

Geometrical isomers These configurations require different groups on each end of the double bond Different forms are called geometrical isomers 45 AB CD BA CD AB CC BA CC Top ofp-orbital viewed from above Two faces Geometrical isomers Equivalent isomers

Shapes of alkynes Because each carbon is sp hybridised (hybrid orbitals 180° apart), ethyne is a linear molecule. Pi bonds form a barrel of electron density around the CC bond. 45

Naming alkenes Suffix "ene" is used to denote alkenes. Number from end nearest double bond (giving first carbon of alkene the lowest number) The stem is based on the longest chain containing the double bond Geometrical isomers are identified using the E–Z nomeclature system 46

Naming alkenes 46 Examples: CH Butene CCHCH 3 CH propyl-2-heptene Butene CH 3 CHCH 3

Naming alkynes Suffix "yne" is used to denote alkynes. Number from end nearest triple bond (giving first carbon of alkyne the lowest number) The stem is based on the longest chain containing the triple bond Linear — no geometrical isomerism 46

Naming alkynes ethyne H–C  C–H propyne CH 3 –C  C–H 1-butyne CH 3 –CH 2 –C  C–H 2-butyne CH 3 –C  C–CH 3 46 Examples:

The E–Z system Distinguishes cis and trans geometrical isomers A group on each end is given preference using rules If these are on the same side — cis — we have the Z-form. "Z" stands for "zusamen" If these are on opposite side — trans— we have the E-form. "E" stands for "entgegen" 47 CC CH 3 H 3 H CC 3 H H 3 transcis and

The E–Z system 47 Must have different substituents at each end There is only one form of 2-methyl-2-butene CH 3 H 3 3 H methyl-2-butene

Prioritising groups 48 1.Priorities based on atomic NUMBER of attached atoms 2.If same atoms are attached compare the attached atoms next along the chain 3.Double and triple bonds are treated as follows:

E/Z designation 48 CH 3 H 3 H Z-2-butene (cis) CH 3 H H 3 E-2-butene (trans) E—priority groups on the opposite face Groups assigned priority based on atomic number of attached atom — CH 3 has priority over H Z—priority groups on the same face

Oxygen and carbon have priority over hydrogen Chlorine and carbon have priority over hydrogen The E–Z system 48 CH 3 2 H ClH CH 3 2 H HCl 1-chloro-1-butene Cis(Z) Trans(E) HOH HCH 3 HOH HCH 3 1-propenol Cis(Z)Trans(E)

HO has priority over methyl and carbon has priority over hydrogen Chlorine and ethyl have priority over hydrogen and methyl The E–Z system 48 CH 3 2 CH 3 ClH CH 3 2 CH 3 HCl 1-chloro-2-methyl-1-butene (Z)(Z) (E)(E) HOCH 3 HCH 3 CH 3 OH HCH 3 2-buten-2-ol (Z)(Z)(E)(E)

The E–Z system 48 The double bond has priority over methyl Methyl has priority over hydrogen Priority groups are on the same side (Z)-alkene

Cycloalkenes Alkene carbon is numbered "one" Number along double bond around ring to the nearest substituent 49

Seeing is believing 50 Movies from Saunders General Chemistry CD-ROM Note the isomerisation is actually from cis to trans