C14 - Alkenes Flashcards
What are alkenes?
Alkenes are unsaturated hydrocarbons, and involves a double covalent bond between carbons
What is Z isomerism in alkenes?
When the higher priority substituents are on the same side of the plane of the C=C bond the isomer is a Z isomer
How are the higher priority substituent groups determined?
The sum of the atomic numbers of the atoms directly attached to this atom decide the priority – higher atomic number means higher priority, if there is a double bond the attached atom must be added twice
What is E isomerism in alkenes?
When the higher priority substituents are on opposite sides of the plane of the C=C bond the isomer is an E isomer
What happens to the boiling point of alkenes as chain length increases and why?
The boiling point of alkenes increases with chain length due to increasing van der Waals forces between molecules
What are electrophiles?
An electron pair acceptor. They are electron deficient
What is electrophilic addition?
Electrophilicadditionis the addition of an electrophile to a double bond
What is the mechanism of electrophilic addition?
In the first stage, the positive charge on the electrophile is attracted to the electron density in the double bond. As the electrophile approaches the double bond, electrons in the A–B bond are repelled towards B. A bonds to the carbon, forming a carbocation. The two electrons in the A–B bond move to B forming a B- ion. The B- ion acts as a nucleophile and attacks the carbocation. The lone pair of electrons on the B- ion is attracted towards the positively-charged carbon in the carbocation, causing B to bond to it.
What is a primary carbocation?
Primary (1°) carbocations have one alkyl group attached to the positively-charged carbon
What happens to the stability of carbocations with the number of alkyl groups?
The stability of carbocations increases as the number of alkyl groups on the positively-charged carbon atom increases, as the more R groups, the less positively charged the carbocation becomes. So tertiary more stable than primary
What is a major product in electrophilic addition?
The major product is the more stable carbocation and the minor is the less stable
What is the test for unsaturation?
Bromine water will turn from orange to colourless if an alkene is present
CH2=CH2 + Br2 + H2O → CH2BrCH2Br + H2O
How are polymers represented?
The polymer can be represented by showing the repeating unit with square brackets around it. The n stands for a unspecified number of monomer units
What is LDPE and how is it formed?
Low-density polyethene (LDPE) is formed under a high pressure (1400 atm) and a temperature of about 170 °C. These conditions cause a high level of branching, meaning that the polymer chains cannot pack tightly together
What is HDPE and how is it formed?
High-density polyethene (HDPE) is formed with a catalyst, a pressure of 2 atm and a temperature of about 70 °C. Little branching occurs under these conditions, resulting in chains that can pack tightly together to create a denser material
