Chemistry of Alkenes Flashcards
What are alkenes
Compounds with a C=C bond
What does -ene mean
One double bond
What does -diene mean
2 double bonds
What does -triene mean
3 double bonds
What is the electronic structure of double bonds (π bonds)
In a π bond p-orbitals of C atoms overlap. π electrons are further from the nucleus (less tightly held) so are involved in the chemistry
Why does cis/trans isomerism exist
Rotation would break the π bond
Describe cis isomers
Z-isomerism (on the same side)
Describe trans isomers
E-isomerism (on different sides)
Out of cis and trans-isomers which form exists more commonly and why
Trans-isomers as there is less steric strain meanign they are more stable
Why are cis isomers less stable
There is repulsion between the methyl groups in the cis isomer.
What types of geometric isomers must be present in (1-7) carbon rings
The double bond must be cis.
In which types of fatty acids do alkenes exist
Unsaturated fats and fatty acids (saturated fats have no C=C bonds)
What type of fatty acids are mammals unable to produce
Fatty acids with C=C double bond further away from carbonyl groups 9 and 10 (counting from the C=O end). These essential fatty acids must be obtained from the diet.
What does omega refer to
The position of the double bond from the CH3 end
What are food sources of linoleic acid
Oily fish, some plant oils
What are fats
Esters of fatty acids
What are terpenes
Hydrocarbons with multiples of 5 carbon atoms
What are terpenes produced by and what is their significance
Produced by many plants. The smaller ones (10 and 15 carbons) are often aromatic and used in flavours and fragrances
What are carotenoids
Brightly coloured compounds containing many conjugated alkenes
Where are carotenoids found
In many plants
What does cojugation mean
Alternating single and double bonds
What can humans convert carotene into
Vitamin A
What is vitamin A required for
Vision in the rod cells of the retina
What do addition reactions of alkenes do
Converts the double bond into 2 single bonds (opposite of elimination)
What happens in addition of hydrogen halides with alkenes
H+ is an electrophile. Electrophiles are attracted to electrons which are found in the double bond. So this is electrophilic addition.
E.g. ethene + HCl -> secondary alkyl chloride
What is Markovnikov’s rule
When there is more than one possible produce, the more substituted halide is usually formed
What happens in hydration of alkenes
The addition of water to give an alcohol. Ethene (+ water and acid catalyst for protons) -> ethanol. This is the reverse of elimination of an alcohol, is acid catalyst and follows “Markovnikov’s rule”
Why is hydration of alkenes reversible
Aqueous acid (lots of water) favours hydration, but conc acid favours dehydration (=elimination). Application of Le Chatelier’s principle
What happens in addition of halogens to alkenes reactions
Br2/ Cl2/ I2 + ethene -> dialkyl bromide/ chloride/ iodide
What if the addition of bromine to alkenes takes place when bromie is dissolved in water
Product is called bromohydrin (CHBrCH2OH). Ethene and water and bromide go to bromohydrin and hydrogen bromide
What is a test for alkenes
The disappearance of the orange/brown colour of the bromide indicated the presence of an alkene.
What can the reaction of unsaturated fatty acids with iodine be used for
To quatify the number of C=C bonds. The “iodine value” is the number of grams of iodine reacting with 100g of oil
What is hydrogenation
Addition of H2 (reduction) to an alkene. The hydrogen molecule contains two electrons. Alkene + hydrogen + metal catalyst -> alkane
What catalyst is used in hydrogenation of alkenes
A solid metal such as Ni. Pd or Pt. The reaction takes place in the metal surface. Metal powder is very fine/ porous or distributed as small particles on carbon to give a large surface area for the reaction
What kind of reaction is the hydrogenation of an alkene
Its a heterogenous reaction (more than one phase)
Why is hydrogenation of alkenes industrially important
It is used for hydrogenating liquid vegetable oils to make solid fats
What is a polymer
A large molecule made up of a repeating unit
How are polymers produced
Through addition of an alkene many times to itself. They are produced by a chain reaction with initiation, propagation and termination steps. Monomers add on to the end of a growing polymer chain
What does polymerisation of ethene produce
Polyethene
What does polymerisation of vinyl chloride produceq
PVC poly(vinyl chloride). Used for pipes, flooring
What does polymerisation of styrene produce
Polystyrene. Used for packaging, cartons, cups
What does polymerisation of tetrafluoroethylene produce
Teflon. Used for non-stick surfaces, cable insulation
What is important to note about polymers in terms of recyling
The code on the plastic packaging indicated the type of polymer
What happens during initiation of polymerisation
Creates radicals from molecules with paired electrons
What happens during propogation of polymerisation
Radical end of polymer chain reacts with alkene, increasing the length of the polymer
What happens during termination of polymerisation
Reaction of two radicals to create a non-radical
