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