Module 4 Chapter 13 Flashcards
what are aliphatic molecules
Are linear or branched open chain structures. Aliphatics may be saturated or unsaturated hydrocarbons with double bonds.
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 line joining the nuclei of the bonding atoms.
What does the pi bond do
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 haloalkene
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 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)
What is a characteristic of the reaction
Sometimes more than one type of product is formed (unsymmetrical or symmetrical)
What is the word equation of the hydration of alkenes
Alkene + water (g) + (phosphoric acid catalyst) —> alcohol
What does the steam do in the reaction and what is it used for in industry
It adds across the double bond - an addition reaction widely used in industry to make ethanol.
What is an isomer
The same structural formula but a different arrangement of the atoms in space
When does E/Z isomerism occur
It only occurs when a c=c double bond is present
Why does E/Z isomerism occur
It occurs when the rotation around the double bond is restricted
So groups attached to either side are fixed relative to each other
If a molecule has both a c=c bond and different groups attached to each carbon atom of the double bond then E/Z isomerism will occur.
When is it a Z isomer
It is a Z isomerism when the groups that have the biggest atomic number on the carbon are on the same side
What is Cis-Trans isomerisation
It is a special case of E/Z isomerism, where one of the attached groups on each C=C must be hydrogen
When is it a cis isomer and when is it a trans isomer
Hydrogen atoms on the same side (z/cis)
Hydrogen atoms on diagonally opposite sides (e/trans)
What is Electrophilic addition
When alkenes undergo addition reactions to form a saturated compound
What does the double bond represent and do
The double bond in an alkene represents a region of high electron density because of the pi electrons. This highly electron density attracts electrophiles.
What is an electrophile
An atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair. Usually they are a positive ion or a molecule with a partially positive charge.
Draw the mechanism for the Electrophilic addition of hydrogen bromide to an alkene (general)
What are carbocations
They are positive ions, with the positive charge on the carbon ion.
Why are carbocations unstable
Because they contain a positively charged carbon atom which creates an electron deficiency
Draw the mechanism for the Electrophilic addition of bromine to an alkene (general)
What does Markownikoff’s rule show
How to predict which of two possible isomers is the major product
What are the three different types of carbocations
Primary carbocations, secondary carbocation, tertiary carbocation
What is the most stable carbocation and why
The most stable carbocation is the tertiary carbocation as they have the most carbon chains attached to them, so there are more electrons present to stabilise the carbocation.
What will the major product be using markowniff’s rule
The major product will be the reaction with the carbocation that is the most stable
Draw the mechanism for the Electrophilic addition of an acid
DRAW IT
Draw the mechanism for the Electrophilic addition of water
DRAW IT
