Alkenes Flashcards
What are alkenes?
Unsaturated hydrocarbons
State what is meant by the term electrophile
An electron pair acceptor
Name the reaction that occurs when an alkene is turned into an alkane (eg C2H4 + HBr – > C2H5Br)
Electrophilic addition
Describe and explain the electrophilic addition of HBr to C2H4
The HBr molecule = polar (Br = more electronegative) –> H = electrophile
C=C makes this area v. negative
Opposites attract - electrophile attracted to double bond
However, Br slightly -ve, so is repelled and breaks off - takes the electrons
The electrophile (H) accepts electrons from a C and bonds –> other C now +ve (carbocation)
Carbocation attracts -ve Br - bonds
Why are major and minor products formed?
Some carbocations are more stable and so are more likely to be formed, more likely to hang around and bond with Nu (and vice versa)
What is a tertiary carbocation?
A C with 3 C groups around
State the stability of carbocations from least stable to most stable
methyl, primary, secondary, tertiary
What is the positive inductive effect?
Alkyl groups push electrons away from themselves towards C+
Why can electrophilic addition by Br2 occur?
High electron density repels the electrons in the Br2 as it nears, inducing a dipole
Electrophilic addition of what allows sulfuric acid to be reformed? What is the other product?
H2O (so hydrolysis reaction) to alkyl hydrogen sulfates
An alcohol is also produced
State what is meant by a polymer
Long chain molecule made from lots of small molecules joined together
State what is meant by a monomer
Small molecules that join together to make polymers
State what is meant by addition polymerisation
Formation of long chain molecules from lots of small molecules together with no other products
What are the conditions for addition polymerisation?
Pressure, high temperature, catalyst
Describe and explain addition polymerisation
Addition of something causes the double bond to break
1C is happy, but now 1 is missing a bond, so ‘steals’ one from the next molecule (causing that double bond to break)
Though this C is now happy, the 1C is not and is missing a bond and so ‘steals’ one from the next molecule, bonding with it…and so on
Plastics are not biodegradable. This means what?
They cannot be broken down by enzymes, acids or alkalis
Why are plastics non-biodegradable?
To break down poly(alkanes) you have to break a non-polar C-C bond. Most bonds are broken down because they are polar or by hydrolysis (polar molecule itself)
What is a plasticiser?
A small molecule that gets between the polymer chains, allowing them to slide past each other. This makes them more flexible
State the meaning of the term stereoisomers
Same molecular and structural formula but different spatial arrangement
A chemical test can be used to distinguish between saturated and unsaturated compounds. State the reagent, and what you would observe with each
Bromine
Saturated - stays orange
Unsaturated - Turns colourless
The formation of ethanol from ethene uses conc. H2SO4 and proceeds in 2 stages.
S1 C2H4 + H2SO4 –> C2H5OsO2OH
S2 C2H5OSO2OH + H2O –> H2SO4 + C2H5OH
State the overall role of sulfuric acid in this process
It acts as a catalyst
What are some typical uses of PVC poly(chloroethene)?
Depending on the type of PVC, can be used in underground pipes, or packaging
