Alkenes

AS Chemistry

• Understand that alkenes and cycloalkenes are unsaturated hydrocarbons.

• Explain the overlap of adjacent p-orbitals to form a π-bond.

• Explain the trigonal planar shape around each carbon making up the C=C bond of alkenes.

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Basic concepts

• Alkenes are unsaturated hydrocarbons with at least one C=C double bond.

• Aliphatic alkenes with one double bond have the general formula: CnH2n

• Alkenes are more reactive than alkanes• Take part in Addition reactions• Alkenes can form E/Z including cis-trans

isomers

Naming of Alkenes

• Select the longest chain of C atoms containing the double bond– End in ENE

• Number the chain starting from the end nearer the double bond– Use a number to indicate the first carbon in the double

bond• Prefix with substituents, side chain positions are

based on the number allocated to the first carbon in the double bond

Ethene

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Covalent bonds are formed by overlap of orbitals.

An sp2 orbital from each carbon overlaps to form a single C-C bond.

The resulting bond is called a SIGMA (δ) bond.

THE STRUCTURE OF ALKENES

The two 2p orbitals also overlap to form a second bond. This is known as a PI (π) bond.

For maximum overlap and hence the strongest bond, the 2p orbitals are in line.

This gives rise to the planar arrangement around C=C bonds.

THE STRUCTURE OF ALKENES

two sp2 orbitals overlap to form a sigma bond between the two carbon atoms

ORBITAL OVERLAP IN ETHENE - REVIEW

two 2p orbitals overlap to form a pi bond between the two carbon atoms

s orbitals in hydrogen overlap with the sp2 orbitals in carbon to form C-H bonds

the resulting shape is planar with bond angles of 120º

Nature of the double bond

• The π-bond fixes the carbon atoms in position– Preventing any rotation of the bond.

• In alkenes, each carbon involved in the double bond– Uses three of its electrons in the formation of 3 σ-

bonds– And uses one of its electrons in the formation of a

π-bond

Shape of an ethene molecule • Three regions of electron density surround each carbon atom in the double bond

• Pairs of electrons repel each other as far apart as possible

• The electron pairs repel each other to give a trigonal shape

• Ethene is a planar molecule, all atoms are in the same plane.

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Cyclohexene • Cylic alkenes have closed rings of carbon atoms containing one or more double bonds

• Most common is cyclohexene

• Do not follow the same general formula as aliphatic alkenes

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•Sometimes used in exam questions•Treat them in the same way as aliphatic alkenes because they react the same way

Shapes of alkenes

• Key words to use– Trigonal – Planar– 120° bond angle– Electron pair replusion

Carotene, C40H56

Week 15

© Pearson Education Ltd 2008This document may have been altered from the original

•How many double bonds?

•Aliphatic or cyclic?

Class work

• Complete questions on page 127• Complete Naming Alkenes Worksheet

Answers to Questions

1. C4H8

2. A)

2. B)

3. E

Z

Answers to Questions

4. a) 2-methylbut-2-eneb) hexa-2,4-diene

Worksheet