3.4.1 structure, bonding and reactivity

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9 Terms

1
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what is the general formula for alkenes?

what is the functional group for alkenes?

  • CnH2n

  • C=C bond

2
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name a functional isomer of alkenes

cycloalkanes, also have general formula CnH2n

3
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are alkenes saturated or unsaturated hydrocarbons?

why?

  • unsaturated

  • because they contain at least one C=C bond

4
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what is the bond angle around the C atoms of a C=C bond?

  • 3 bonding pairs and no lone pairs around each C atom.

  • so trigonal planar.

  • each bonding pair repels as far apart as possible.

  • the double bond repels slightly more than the single bonds.

  • bond angle is around 120 degrees.

5
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what is a sigma bond?

  • formed when electron orbitals directly overlap

  • bond is freely rotational

6
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what is a pi bond?

  • formed when electron orbitals overlap sidewards

  • bond is not freely rotational, as in the case of alkenes any rotation would reduce the overlap of the p orbitals.

7
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explain the bonding in alkenes

the double bond in alkenes consists of a sigma bond and a pi bond.

  • when the double bond forms, a sigma bond forms between the C atoms.

  • each C atom now has one electron left in a p orbital.

  • the two p orbitals overlap sideways, above and below the plane of the molecule.

  • this forms a pi bond, which is above and below the sigma bond.

8
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how does the bonding in alkenes affect their structure?

  • the pi bond cannot rotate freely, as any rotation would reduce the overlap of the p orbitals.

  • this causes a lack of rotation around the C=C bond.

  • this means the structure of an alkene across the double bond is locked.

  • this allows alkenes to form E-Z stereoisomers.

9
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how does the bonding in alkenes affect their reactivity?

  • the double bond in alkenes is a centre of high electron density, as there are 2 pairs of electrons.

  • this means alkenes are susceptible to attack from electrophiles.

  • this makes alkenes highly reactive molecules.

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  • pi bonds have a lower bond enthalpy than sigma bonds and are easier to break, as in a pi bond the orbitals overlap sideways, whereas in a sigma bond the orbitals directly overlap.

  • so, pi bonds are more likely to take part in reactions.

  • this makes alkenes highly reactive molecules.