Module 4 Chapter 13
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what are aliphatic molecules
Containing carbon atoms joined together in unbranched (straight) or branched chains and are not aromatic
What is the general formula of alkenes
CnH2n
What can alkenes be
They can be branched and contain more than one c=c double bond or be cyclic
What do cyclic alkenes and alkenes with more than one c=c bond do
Cyclic alkenes and alkenes with more than one c=c don’t follow the general rule
What can each carbon form
They can make 4 covalent bonds as they have 4 electrons present
What are 3 of the electrons used in and where are the bonds found
3 of the electrons are used in 3 sigma bonds Where: 1 is to the other carbon in the double bond 2 are to two other atoms (c or h)
What is 1 electron in the carbon - carbon double not involved in and what orbital is it in
1 electron on each carbon atom of the double bond is not involved in sigma bonding This electron is in a p orbital
What is a pi bond
It is formed by the sideways overlap of two orbitals, one from each carbon in the double bond
What does each carbon involved int eh double bond do
It contributes one electron to the electron pair in the pi bond.
Where is the electron density in the pi bond concentrated
Above and below the sigma bond.
What does the pi bond do (rotation)
It locks the 2 carbons of the C=C in position and thus, prevents them from rotating around the double bond
What is the shape of the molecule around the carbon atoms in the double bond
It is trigonometric planar as there are 3 regions of electron density. They repel as far apart as possible - so the bond angle is around 120*.
What are alkenes used in
The production of plastics - polymers Some alkenes regulate fruit ripening - ethene Pharmaceutical synthesis Used as fuels - found in diesel Used to make detergents and can be converted to alcohols.
What are the possible reactions of alkenes
Combustion Electrophilic addition includes Hydrogen Bromine Hydrogen bromides Steam Additional polymerisation (free radicals)
What is the reactivity of alkenes
They are more reactive than alkanes due to the pi bond
Why are alkenes more reactive than alkanes due
C=C bond is made up of a sigma bonding and a pi bond The pi bond electron density is concentrated above and below the plane of the sigma bond The pi electrons are more exposed Therefore the pi bond more readily breaks and alkenes undergo addition reactions relatively easily
What occurs in Electrophilic addition
A small molecule is added across the double bond, the pi bond breaks and new bonds form
What is the word equation for the hydrogenation of alkenes
Alkene + hydrogen (+ nickel catalyst) —> Alkane
What are the conditions and catalyst required for the hydrogenation of alkenes and what occurs in the reaction
423K and a nickel catalyst Hydrogen is added across the double bond = addition = hydrogenation
What is the halogenation of alkenes
it is where chlorine or bromine at room temperature is added across the c=c bond to form a haloalkane
How can halogenation be used to differentiate between alkenes and alkanes
Add orange bromide water drop wise to a sample of an alkene, the bromine is added across the double bond and the colour disappears. This tells us a c=c bond is present. Whereas if it was done for an alkane there would be no change in colour.
What is the reaction conditions of the addition addition of hydrogen halides to alkenes
Alkenes react with gaseous halides at room temperature forming haloalkanes. They are both gases when you mix them together.
How are the hydrogen halides added if the alkene is a liquid
Then the hydrogen halide is bubbled through
What do alkenes also react with (acid)
They also react with concentrated HCl or HBr (hydrobromic acid)