13.1 & 13.2 - Properties of Alkenes and Stereoisomerism

Bee Brittain
Mind Map by Bee Brittain, updated more than 1 year ago
Bee Brittain
Created by Bee Brittain about 4 years ago
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AS - Level Chemistry (13 - Alkenes) Mind Map on 13.1 & 13.2 - Properties of Alkenes and Stereoisomerism, created by Bee Brittain on 03/10/2016.

Resource summary

13.1 & 13.2 - Properties of Alkenes and Stereoisomerism
1 Structure
1.1 Alkenes and cycloalkenes are UNSATURATED HYDROCARBONS
1.1.1 Contain at least one Carbon-Carbon double bond
1.2 Each C atom has 4 electrons in its outer shell
1.2.1 For each C atom in double bond:
1.2.1.1 3 of these four electons are used in 3 sigma bonds
1.2.1.2 One goes to form a double bond with the other Carbon
1.2.1.3 Other two electrons go to two other atoms (carbon or hydrogen)
1.2.1.3.1 The pi bond is formed by the sideways overlap of two p-orbitals
1.2.1.3.1.1 The pi-electron density is concentrated above and below the line joining the nuclei of the bonding atoms
1.2.1.3.1.2 Pi bond locks the C atoms in position and prevents them from rotating
1.2.1.4 The Shaping Around A Double Bond
1.2.1.4.1 3 regions of electron density around each C atom
1.2.1.4.2 the three regions repel each other as much as they can

Annotations:

  • This is because electron density is symmetrical around the molecule so the repulsion is also symmetrical.
1.2.1.4.2.1 120 degree bond angle

Annotations:

  • Because bonding angle is 120 degrees, this means the shape of ethane for example, is like two trigonal planars facing each other with an overlap of one prong.
1.2.1.4.3 All atoms in the same plane
2 Stereoisomerism
2.1 Same structural formula, different arrangement of atoms in space
2.2 E/Z Isomerism
2.2.1 Only occurs in compounds with a C=C double bond
2.2.2 Has to have different groups attached to each carbon atom of the double bond
2.2.3 E.g// The E/Z Isomers of but-2-ene
2.2.3.1
2.2.3.1.1 E-But-2-ene
2.2.3.2
2.2.3.2.1 Z-but-2-ene
2.2.4 Cis-trans Isomerism Is a special case of isomerism
2.2.4.1 Its special because in cis-trans Isomerism, one of the attached groups on each carbohn atom of the double bond MUST be hydrogen
2.3 Optical Isomerism
2.3.1 Can occur in a wide range of compounds, including alkanes with no functional groups
3 Cahn-Ingold-Prelog Nomenclature
3.1 If the groups of higher priority are on the same side of the double bond
3.1.1 Z ISOMER
3.2 If the groups of higher prority are diagonally places across the double bond
3.2.1 E ISOMER
3.3 Assigning Priority
3.3.1 STEP ONE: Examine atoms attached directly to the carbon atoms, decide which has highest priority. The higher the atomic number, the higher the priority
3.3.1.1 STEP TWO: If the two atoms attached to a carbon atom in the double bond are same, you'll need to find the first point of difference. The group which has the higher atomic number at the first point of difference is given the higher priority.
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