SNS - Organic Chemistry - Alkenes and Alkynes Flashcards
Alkenes
Physical Properties
- Melting Points
- Boiling Points
- Increase with increasing molecular weight, similar in value to corresponding alkanes. Trans-alkenes usually have higher melting points than cis alkenes due to increased symmetry which allows for better packing in the solid state
- Increase with increasing molecular weight, similar in value to corresponding alkanes. Terminal alkenes (1-alkenes) usually boil at a lower temp than internal alkenes and can thus be separated by fractional distillation. Trans-alkenes tend to have lower boiling points that cis-alkenes as are less polar
Alkenes
Physical Properties
Polarity
Results from the assymetrical distribution of electrons in a particalar molecule
In alkenes, creates a dipole moment oriented from the electropositive alkyl groups towards the electronegative alkene
Net dipole of alkene compounds can be assessed by the distribution of electrons across the molecule
Polarity created additional intermolecular forces which act to raise boiling point
Alkenes
Physical Properties
Polarity
Dipole moments from the two electropositive alkyl groups create a net dipole moment
The molecule is polar
Alkenes
Physical Properties
Polarity
Dipole moments created by the two electropositive alkyl groups are oriented in opposite directions and cancel out
Not net dipole, molecule is non-polar
Alkenes
Synthesis
Can be synthesized in a number of ways, most commly by elimination reactions of either alcohols or alkyl halides
In these reactions, the carbon skeleton loses HX (where X is a halide) or a molecule of water to form a double bond
Elimination occurs by two distinct mechanisms: E1 and E2
Alkenes
Synthesis
Mechanism
E1
Unimolecular elimination - first order - rate of reaction depends only on the concentration of the substrate
Two step process proceeding through a carbocation intermediate. The elimination of a leaving group and a proton results in the production of a double bond
- The leaving group departs producing a carbocation
- A proton is removed by a base
Alkenes
Synthesis
E1
Conditions
Favoured by the same conditions as for S1:
- Highly polar solvents
- Highly brnched carbon chains
- Good leaving groups
- Weak nucleophiles
- Low concentrations of nucleophiles
E1 and S1 therefore competetive. Driving a reaction towards either S1 or E1 in particular is difficult, although E1 is favoured by a higher temp
Other factors such as the polarity of the solvent and branching of the carbon chain can be modified to reduce the competition between E1 and S1
Alkenes
Synthesis
Mechanism
E2
Single step - second order - rate is dependent on concentrations of substrate and base
A strong base such as ethoxide ion (C2H5O-) removes a proton, while a halide ion anti to the proton leaves resulting in the formation of a double bond
Often there are two possible products. In these cases, the more substituted double bond is formed preferentially
Alkenes
Synthesis
E2
Condtions
Controlling E2 vs S2 is easier than controlling E1 vs S1:
- Steric hindrance doesn’t greatly affect E2. Therefore highly substituted carbon chains, which form the most stable alkenes undergo E2 most easily and S2 rarely
- A strong base favours E2 over S2. S2 is favoured over E2 by weak Lewis bases (strong nucleophiles)
Alkenes
Reactions
- Reduction
- Electrophilic Addition - (a) HX, (b) X2, (c) H2O
- Free Radical Addition
- Hydroboration
- Oxidation - (a) Potassium Permanganate, (b) Ozonolysis, (c) Peroxycarboxylic Acids
- Polymerisation
Alkenes
Reaction
Reduction
Catalytic hydrogenation is the process of adding molecular hydrogen to a double bond with the aid of a metal catalyst
Reaction takes place on the surface of the metal. One face of the double bond is coordinated to the metal surface, thus the two hydrogen atoms are added to the same face of the double bond - syn addition
Alkenes
Reaction
Reduction
Typical catalysts are
- platinum,
- palladium
- nickel
Also,
- rhodium,
- iridium,
- ruthenium
Alkenes
Reactions
Electrophilic Addition
The pi bond is somewhat weaker than the sigma bond and can therefore be broken without breaking the sigma bond.
Though many addition reactions exist, most operate via the same mechanism – the electrons of the pi bond are particularly exposed and thus easily attacked by molecules seeking to obtain an electron pair (Lewis acids) - electrophiles
- HX Addition
- X2 Addition
- Addition of H2O
Alkenes
Reactions
Electrophilic Addition
HX
The electrons of the double bond act as a Lewis base and donate electrons to HX molecules.
In cases where the alkene is assymetrical, the initial protonation proceeds to produce the most stable carbocation - the proton will add to the less substituted carbon atom (carbon atom with the most protons) since alkyl substituents stabilise carbocations - Markovnikov’s Rule
- Double bond reacts with a proton to yield a carbocation intermediate
- The halide ion combines with the carbocation to give an alkyl halide
Alkenes
Reactions
Electrophilic Addition
X2
The addition of halogens to a double bond is a rapid process
- Double bond acts as a nucleophile and attacks the X2 molecule displacing X-.
- The intermediate carbocation forms a cyclic halonium ion which is then attacked by X- to yield the dihalo compound. This addition is anti as the X- attacks the cyclic halonium ion in a standard SN2 displacement
If the reaction is carried out in a nucleophilic solvent, the solvent molecules can compete in the displacement step to produce, for example, a halo alcohol (rather than the dihalo compound)
