Chapter 13 - Alkenes (MODULE 4) Flashcards
What are alkenes
unsaturated hydrocarbons with a C=C double bond
What is the general formula of aliphatic alkenes with one C=C bond
Cn H2n
What are the first three alkenes
ethene, propene, but (1/2)ene
How are the electrons used in a Carbon which is part of a C=C bond
3 sigma bonds, 1 pi bond
How are pi bonds formed
sideways overlap of two p orbitals
Where is the pi bond electron density
above and below the plane
What is different about stereoisomerism in alkanes and alkenes
Alkenes cannot rotate about the carbon whilst alkanes can, meaning alkenes have stereoisomers
What is the shape and bond angle around a carbon with a double bond
- why
120 degrees, trigonal planar
- three electron-dense regions that repel each other as much as possible
What is limolene resposible for
the smell and flavour of citrus fruits e.g. oranges and lemons
What are stereoisomers
isomers with the same structural formula but different arrangements of atoms in space
What type of compounds to E/Z isomers occur in
compounds with a C=C double bond
Why does stereoisomerism arise around double bonds
because rotation about the double bond is restricted and therefore groups attached to the carbon are fixed relative to each other. This is because the pi bond electron density is above and below the sigma bond plane
What two conditions are needed for E/Z isomerism
- there is a C=C double bond
- different groups are attached to each carbon of the double bond
What additional condition is needed for cis/trans isomerism
one of the attached groups on each carbon atom of the double bond must be the same
Which isomer (E/Z) is
- cis
- trans
- cis is Z isomer
- trans is E isomer
What do the Cahn-Ingold-Prelog laws state
groups attached to each carbon atom in a double bond are given priorities (1 or 2)
- if the higher priority groups are on the same side it is a Z isomer. If they are on opposite sides it is an E isomer
Using the Cahn-Ingold-Prelog rules, how do we assign priority.
Check the first atom directly attached to the carbon chain. The highest atomic number gets priority. If they are the same, then check the next atom and so on
Why are alkenes much more reactive than alkanes
because of the presence of the pi bond. As the electrons are outside the plane, they are more exposed and so the pi bond breaks easily
What four types of substances do alkanes undergo addition reactions with
hydrogen
halogens
hydrogen halides
steam
What are the conditions, reactants and products of hydrogenation of alkenes
alkene + H2 -> alkane
passed over a nickel catalyst at 423 K
What are the conditions, reactants and products of halogenation of alkenes
alkene + X2 -> dihaloalkane
What is the test for unsaturation
add bromine water. If unsaturated, solution will go from yellow to colourless
What are the conditions, reactants and products of the reaction of alkenes with hydrogen halides
alkene + HX -> haloalkane
if alkene is gas, mix the two compounds
if the alkene is liquid, bubble HX through it
If the alkene is unsymmetrical, use Markovnikov’s rule to determine major and minor product
What are the conditions, reactants and products of hydration of alkenes
alkene + steam -> alcohol
water as steam, phosphoric acid catalyst, and in excess of 100 degrees Celsius (373 K)
