Org 4 - Alkenes Flashcards
What is the general formula of an Alkene ?
CnH2n for straight chain
2 Hydrogens are subtracted for each ring
What is the functional group of an Alkene ?
The double bond
What are the physical properties of Alkenes ?
- Similar to Alkanes
- They may be polar due to the nature of the double bond
- Trans Alkenes have higher melting points due to better symmetry, and lower boiling points due to less polarity
Describe how and why stability occurs in Alkenes ?
The greater the number of attached alkyl groups (ie. The more substituted the double bond), the greater the stability.
What are Alkenes with 2 and 3 double bonds called ?
2 - Diene
3 - Triene
What is a Polyene ?
An Alkene with many double bonds
What is a Conjugated Diene ?
A diene with a single bond between the double bonds - this is usually more stable due to resonance stabilisation.
Which Alkenes do not engage in addition reactions ?
Aromatic compounds (Conjugated double bonds in a ring)
What are the two most common synthesis reactions of Alkenes ?
Elimination reactions of either HX (from an alkyl halide) or H2O (from alcohol)
What are the four most important chemical reactions of Alkenes ?
1 - Electrophilic addition
2 - Oxidation
3 - Hydrogenation
4 - The Diels-Alder reaction
Name and describe two common Electrophiles
Electrophiles - Substances that seek electrons (have a positive charge)
H+ in HBr, H2O etc…
Name and describe two common Nucleophiles
Substances which seek protons, they have a free pair of electrons (and sometimes a negative charge).
E.g. OH-, CN-
Describe stability in Carbocations
Primary Carbocations are the least stable, tertiary the most. Carbocations are stabilised by nearby electron-donating groups.
When do you use Markovnikovs rule ?
To predict which carbon will become the carbocation, and to which carbon the Electrophile and Nucleophile will bond in a Nucleophilic reaction.
What is Markovnikov’s rule ?
The Nucleophile will be bonded to the most substituted carbon in the product.
Equivalently, the Electrophile will be bonded to the least substituted carbon in the product.
E.g. “Hydrogen wants to be with its friends”
In Nucleophilic addition, what is the product called if the H+ bonds to the most and least likely carbon ?
Most likely - Markovnikov’s product
Least likely - Anti-Markovnikov’s product
Name a simple laboratory diagnostic test for the presence of unsaturation ?
NB: Unsaturation = double or triple bond.
Addition of Halogens e.g. Br2 Colour indicator (Reddish brown –> Clear)
C=C + X2 –> X-C-C-X
This doesn’t occur in hydrocarbons which are saturated as no e- rich double bond
Why does Halogen addition not occur in aromatic rings ?
There is increased stability due to the conjugation of the ring system
Name two common agents used to oxidate Alkenes ?
1 - Potassium Permanganate (KMnO4)
(Under mild conditions - no heat)
2 - Osmium Tetroxide (OSO4)
How does heat affect the Oxidation properties of KMnO4 ?
Under more abrasive conditions (E.g. Heat), Potassium Permanganate and Alkenes lead to Oxidative Cleavage of the double bond
What is Ozonolysis ?
Ozonolysis is when Ozone reacts vigorously with Alkenes
What is the product of Ozonolysis ?
The reaction leads to Oxidative Cleavage of the double bond which can produce a Ketone and an Aldehyde
What is the chemical formula for Ozone ?
O3
What is Hydrogenation of Alkenes ?
Alkenes react with hydrogen in the presence of a variety of metal catalysts (E.g. Nickel, Palladium, Platinum).
This is a Syn addition reaction
Why is the Hydrogenation of Alkenes a Syn reaction ?
Both Hydrogens add to the double bond from the same side as the reaction occurs on the surface of a metal catalyst
Name the reaction type of Alkene Hydrogenation ?
Heterogenous Catalysis, because there are two phases present:
Hydrogenation
The Metal Catalyst
What is another term for Hydrogenation ?
Saturation, as the Carbon atom is bonded four times
What is the effect of Hydrogenation on any other functional groups attached to the Alkene ?
Alkenes are much more reactive than other function groups, as a result, other groups e.g. Ketones, Aldehydes, Esters, and Nitriles are usually unchanged during the Alkene hydrogenation process
What is a Diels-Alder reaction ?
Cyclo-addition reaction between a conjugated diene and a substituted Alkene (The dienophile) to form a substituted Cyclohexane system
What is the generic formula for a Diels-Alder reaction ?
Diene + Dienophile –> Cyclohexene
Explain the four principles of a Diels-Alder reaction ?
- Heat initiated reaction
- Forms a new, 6 membered ring
- 3 Pi bonds break
- 2 new C-C bonds
- 1 new Pi C-C bond formed
- All bonds break and form in a single step (Peri-cyclic)
Describe the Diels-Alder Diene ?
Must have two double bonds on the same side of the single bond in one of the structures (CIS with respect to the single bond)
Why is a Diels-Alder reaction useful ? Name 5 reasons
- It sometimes creates Stereocenters
- It always forms a ring
- The reaction is Stereo-specific
- The reactions are reversible
- It is often used in the synthesis of complex molecules
What is the relationship between the number of resonance forms, and the stability of a molecule ?
The greater the numbers of authentic resonance forms possible, the more stable the molecule
What is an Alkyne ?
An unsaturated Hydrocarbon molecule containing Carbon-Carbon triple bonds
Explain the boiling point trend displayed by Alkynes ?
Higher boiling points than Alkanes or Alkenes.
Internal Alkynes, where the triple bond is in the middle of the compound, boil at higher temperatures than terminal Alkynes
What is the pH of terminal Alkynes ?
Relatively Acidic
In what way do the reactions of Alkynes differ from those of Alkenes or Alkanes ?
Basic reactions such as reduction, Electrophilic addition, free radical addition and hydroboration proceed in a similar manner to Alkenes.
BUT Alkynes can be partially hydrogenated yielding Alkenes.
What is the bond angle in an Alkyne ?
Linear - 180 degrees
Describe the Alkyne bond ?
Linear triple bond, 1 Sigma and 2 Pi bonds
What is an Alkene ?
An unsaturated chemical compound containing at least 1 carbon to carbon double bond
