Summary
Summary Module 4.1.3 Chapter 13 Alkenes
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notes on: 1 - properties of alkenes 2 - stereoisomerism 4 - reactivity of alkenes 5 - electrophilic addition 6 - polymerisation in alkenes 7 - environmental concerns
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13.1 - NOTES = properties of alkenesStructure - unsaturated hydrocarbons
- C=C with at least one C
- CnH2n
- Branched or cyclic (but have 1+ double bond)
Double bond - Each carbon atom has 4 e- in its outer shell and uses electrons to form bonds
- Each carbon atom of double bond – ¾ e- are used in 3 sigma bonds.
- 1 to other carbon atom, other 2 to other atoms (C or H)
- 1 electron is in P orbital.
- A Pi bond is formed by the sideways overlap of 2 p orbitals, 1 from each carbon of the
double bond.
- Each carbon contributes one e- to e- pair in Pi bond.
- Pi electron density is concentrated above and below line joining nuclei of bonding atoms.
- Pi bond locks 2 carbon atoms in position preventing them from rotating around the
double bond.
- Making rotation possible around every atom.
Shape - Trigonal planer
around a - 3 regions of e- density around each C.
double bond - 3 regions repel each other as far apart as possible so it is 120*
13.2 - NOTES = stereoisomerism
stereoisomers - Same structural formula but different arrangement of atoms
- E/Z - in C=C compounds
- Optical – different types including alkanes with no func group.
E/Z - Rotation about double bond is restricted and groups attached to each carbon are fixed
relative to each other.
- Rigidity due to position of Pi bonds e- density above and below of the plane of sigma
bond.
- Must have both double bond and different groups attached to each carbon.
Cis/trans - Special type of E/Z
- Must have C=C and each carbon attached to different groups
- Each C must have H attached.
- Cis – Z isomer (groups on same side)
- Trans – E (groups on alternating sides)
Cahn-Ingold- - used when each C atom is attached to a single H atom
Prelog - Using atomic number to figure out priority group (higher = higher)
- Groups with higher priority is on same side = Z isomer
- Groups with higher priority is on alternating = E isomer
- If groups attached to a carbon are the same (like 2 H) find the first point different.
- C group has more priority over H, in the alkyl or function groups show what makes it
different (change of element)
- The different element with higher atomic number is the chain with higher priority
13.3 - NOTES = reactivity of alkenes
Reactivity - Alkenes more reactive than alkanes due to Pi bond
- C=C has sigma and Pi bond where Pi e- density is concentrated above and below the plane of
sigma bond
- As it is outside of double bond, Pi e- are exposed and readily breaks and alkenes undergo
addition reactions easily.
- A single sigma bond has higher bond enthalpy than Pi bond.
- Therefore, Pi bond is weaker than sigma and broken more readily.
Addition - Hydrogen with nickel catalyst - Each reaction involves addition of small molecule
reactions - Halogens across the double bond.
- Hydrogen halides - Pi bond breaks for new bonds to form.
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