AS Chemistry Chapter 1.7 - Alkenes

Edexcel AS Chemistry notes.

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Chapter 1.7 ­ The Alkenes
In a carbon-carbon single bond, also known as a sigma bond, the electron cloud is symmetrical about
the central axis of the molecule. The sigma bond can rotate about this axis.
The carbon-carbon double bond in ethane involves a sigma bond and a bond, whose electron
density is concentrated on either side of the bond. The bond does not allow rotation.
The electron density in the bond is further away from the nuclei of the carbon atoms so it is
weaker than a sigma bond and easier to break.
Molecules containing double bonds: -
Are reactive due to the attraction of the positively polarised groups to the electron-rich
bond with the double bond. These electron seeking groups are known as electrophiles. The
bond provides an electron pair to form a new bond with the electron seeking group.
They show geometric isomerism. Rotation about the bond axis is not possible without
breaking the bond.
The lack of free rotation around the double bond results in molecules known as geometric isomers.
The traditional way of naming geometric isomers is known as cis-trans isomerism.
E.g. but-2-ene
H H H CH
C C C C
HC CH HC H
Cis-but-2-ene trans-but-2-ene
In cis-isomers the methyl groups are on one side of the bond (both at the top or both at the
bottom), and the H's are on the other side.
In trans-isomers, the 2 methyl groups are on opposite sides to each other.
Geometric isomers have different physical properties, but the chemical reactivity is the same.
The E-Z isomerism system is used to name geometric isomers.
In the E-Z system, the groups attached to the C C bond are ranked based on their atomic number.
If the two higher-priority groups are on the same side (both at the top or both at the bottom) it is a
Z-isomer.
If the higher-priority groups are on opposite sides then in it an E-isomer.
H E.g. but-2-ene H H CH
C C C C
HC Z-but-2-ene HC E-but-2-ene H
CH

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E.g. 1-bromo-2-chloro-2-fluoro-1-iodoethane
Br F Br Cl
C C C C
I Cl I F
From `I' and `Br', the `I' has the higher priority The high priority groups (F and Cl) are
because it's got the higher atomic number. From on opposite sides so it's an E-isomer.
Cl and F, Cl has a higher priority. They are both at
the bottom so it's a Z-isomer.…read more

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Reactions between alkenes and hydrogen halides ­
E.g.
H H H H
C C + HBr H C C H
H H H Br
E.g.
H H H
H H C C H
1-bromopropane
H H H H Br
H H C C + HBr
H H H
H H H
H H C C H
2-bromopropane
H Br H
2-bromopropane is the major product.…read more

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In reactions involving alkenes, the electron pair of the bond froms a new bond with an electrophile
(positively polarised attacking group).
E.g.
H H H H
C C H C C H
H H H Br¯
The 2 electrons in H Br
the bond form
a bond with the
The 2 electrons in
hydrogen atom.…read more

Comments

sopi

THANKS !!! your notes are clear and easy to  remember :)

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