3.3.4 - Alkenes Flashcards
Why is there no rotation around the C=C bond?
The p-orbitals overlap to form an orbital with a cloud of electron density both above and below the single bond (a pi-bond).
Define ‘structural isomerism’.
The same STRUCTURAL formula but different arrangement of atoms in space
Alkenes have ______ bpts/mpts to alkanes - why?
Alkenes have similar bpts/mpts to alkanes - same intermolecular forces (van der Waals’)
With asymmetrical alkenes, the carbocation that forms leads to the major and most stable product. Which carbocation is the most stable and why?
Teritary carbocations are most stable, due to the positive inductive effect
Explain the ‘positive inductive effect’.
Chem Guide:
- Alkyl groups “push” electrons away from themselves, as the other end of the bond attracts the shared electrons more strongly.
- This means that the alkyl group becomes slightly positive and the carbon they are attached to becomes slightly negative
- The importance of spreading charge around in making ions stable: the general rule-of-thumb is that if a charge is very localised (all concentrated on one atom) the ion is much less stable than if the charge is spread out over several atoms.
- The net effect is that the positive charge is being spread out over more and more atoms as you go from primary to secondary to tertiary ions.
- The more you can spread the charge around, the more stable the ion becomes.
Define ‘repeat unit’.
The smallest group of atoms that can produce the polymer when repeated over and over.
Define ‘carbocation’.
Where positive charge is carried on a carbon atom.
Alkenes are ______ hydrocarbons. Their general formula is _______.
Alkenes are unsaturated hydrocarbons. Their general formula is CnH2n.
Are alkenes more or less reactive than alkanes?
More reactive, due to high electron density in C=C bond.
Are alkenes soluble in water?
No!
What shape of molecule is ethene and what is the bond angle?
Planar, 120degrees
What kinds of isomerism occur with alkenes?
- Position isomerism (position of carbon-carbon double bond)
- E/Z isomerism (stereoisomerism)
How do you assign priority in E/Z isomerism?
Cahn-Ingold Prelog notation: the higher atomic number atom has priority (if you have the same groups bonded to each carbon, priority cannot be assigned).
The bond enthalpy for C-C is ____________.
The bond enthalpy for C=C is ____________.
Therefore you would think that the ______ bond would be more reactive.
The bond enthalpy for C-C is 347kJ mol-1.
The bond enthalpy for C=C is 612Kj mol-1.
Therefore you would think that the C-C bond would be more reactive. Not true!
Define the term ‘electrophile’.
An electron pair acceptor.
