4. Alkenes Flashcards
Are alkenes saturated or unsaturated?
Unsaturated
Do alkenes contain a double bond?
Yes
What is the arrangement of atoms around the carbon-carbon double bond in alkenes?
Trigonal planar
What is the bond angle of atoms around the carbon-carbon double bond in alkenes?
120°
What do the p orbitals on each C in the double bond overlap to form?
Two different types of bond; σ or π
What are the two ways that alkenes can react?
- by free radical addition
* by electrophilic addition
How is a pi bond formed?
Due to the overlap of p-orbitals
What gives alkenes chemical properties that are not seen in alkanes?
The C-C double bond
What is an addition reaction?
The combination of two or more molecules to form a single molecule as a result of breaking the double bond
Which type of reaction is faster; addition or substitution?
Addition
Why are addition reactions generally faster than substitution?
Only weak π bonds are broken in addition, rather than strong σ bonds
What is an electrophile?
A species that can accept a pair of electrons from a species with high electron density
What is the area of high electron density in an alkene?
The σ-bond and π-bond
Why can σ and π bonds attract electrophiles?
They are areas of high electron density
In alkenes, after an electrophile is attracted to the σ or π bond, what happens?
The electrophile undergoes heterolytic fission
What is heterolytic fission?
Breaking of a covalent bond which results in both electrons going to the same atom
Why are alkenes much more reactive than alkanes?
They can undergo addition, and are able to react with electrophiles as well as free radicals
What is likely to happen in an alkene, considering that the bond is non-polar?
Homolytic fission may be undergone instead of heterolytic
What are symmetrical alkenes?
Where both carbon atoms in the double bond are attached to the same groups
What is it called when an atom has both carbon atoms in the double bond attached to the same groups?
Symmetrical
What are unsymmetrical alkenes?
Alkenes with the two carbons in the double bond not attached to the same groups
What are possible electrophiles in addition reactions with symmetrical alkenes?
- halogens (e.g. Br-Br)
- hydrogen halides (e.g. H-Br)
- H2SO4
What can symmetrical alkenes react with to undergo an addition reaction?
- halogens (e.g. Br-Br)
- hydrogen halides (e.g. H-Br)
- cold concentrated H2SO4
What are the stages in an addition reaction of a symmetrical alkene?
- delta +ve atom in electrophile attracted to electron rich C=C
- double bond breaks and pair of electrons allow one of the C atoms to bond to the delta +ve atom
- as a result second carbon in C=C is left electron deficient and a carbocation is formed
- rest of electrophile molecule is electron rich with a lone pair and -ve charge - this is attracted to +ve C atom and lone pair used to form a bond (cancelling both charges)
How can bromine react in an addition reaction with a symmetrical alkene, even though it is non-polar?
In the presence of alkenes, electrons move to one side of molecule - acquires temporary dipole
What is formed when Br₂ acts with a symmetrical alkene?
A halogenoalkane
During the reaction with Br₂ and a symmetrical alkene, what is the colour change?
Orange to colourless
Why is bromine water a good test for unsaturation?
When Br₂ reacts with a symmetrical alkene, the double bond must break to react - showing that the alkene is unsaturated
What conditions does the addition reaction between a symmetrical alkene and bromine water take place under?
Takes place readily at room temperature - fairly quickly
Why does hydrogen bromide undergo addition with symmetrical alkenes?
The H in the H-Br bond has a positive dipole and is attacked by the pair of electrons on the C=C bond, which undergoes heterolytic fission
What conditions does the addition reaction between a symmetrical alkene and hydrogen bromide take place under?
Takes place readily at room temperature - fairly quickly
What are the two steps in the addition reaction of alkenes and sulphuric acid?
- Alkenes undergo electrophillic addition with cold concentrated sulphuric acid
- If the mixture is warmed and then water added, the H₂SO₄ group will be replaced by an OH group and alcohol formed
What is formed when ethene undergoes electrophillic addition with cold sulphuric acid?
Ethyl hydrogen sulphate
What is formed when ethyl hydrogen sulphate is warmed and water added?
Ethanol and sulphuric acid
What reaction happens when ethyl hydrogen sulphate is added to water and warmed?
Hydrolysis
What is hydrolysis?
Using water to break covalent bonds
What is the overall equation for the addition reaction and hydrolysis, involving ethene and sulphuric acid?
CH₂CH₂ + H₂O → CH₃CH₂OH
What does the sulphuric acid in the addition reaction and hydrolysis with alkenes act as?
A catalyst
What is the addition reaction and then hydrolysis of alkenes used for in the laboratory?
Useful way of converting alkenes into alcohols
What happens if unsymmetrical alkenes react with unsymmetrical electrophiles?
The atoms in the electrophile can bond to either carbon in the double bond producing a different carbocation - this results in two possible outcomes
What are some examples of unsymmetrical electrophiles?
- H-X
* H₂SO₄
What two products could be formed from the reaction of propene with hydrogen bromide?
- 1-bromopropane
* 2-bromopropane
There are two products formed from the reaction of propene with hydrogen bromide. Are they formed in equal quantities?
No
When an unsymmetrical alkene reacts with an unsymmetrical electrophile, what determines the likelihood of each product being formed?
The stability of the carbocation intermediate
When will a carbocation be more stable?
When there are more alkyl groups attached to the positively charged carbon
Why are carbocations more stable when there are more alkyl groups attached to the positively charged carbon?
Because alkyl groups tend to release electrons, known as the electron releasing effect
What is the electron releasing effect?
When carbocations are made more stable when the positively charged carbon is attached to more alkyl groups due to alkyl groups releasing electrons
What happens when a carbocation is more stable, in terms of how likely it is to be produced?
More stable the carbocation = more likely it is to be produced
What is the major product?
The product that is more likely to be produced
What is the major product formed from?
The more stable carbocation from the two possible intermediates
What is the minor product formed from?
The less stable carbocation
What is the order of stability of carbocations?
1° < 2° < 3°
What can the electron releasing effect sometimes be known as?
The inductive effect
What will be the major product formed in the reaction between propene and hydrogen bromide?
2-bromopropane
What will be the minor product formed in the reaction between propene and hydrogen bromide?
1-bromopropane
Why do more alkyl groups stabilise carbocations?
Alkyl groups contain a greater electron density. This density is attracted towards and reduces the charge on the C atom
What can addition polymers also be known as?
Polyalkenes
What are addition polymers formed from?
Alkenes and substituted alkenes
What are substituted alkenes?
Alkenes where the hydrogens have been replaced by other atoms or groups
What are addition polymers made up of?
Small molecules containing a double bond
How is an addition polymer formed?
- double bond is broken in many monomers, under pressure, forming the repeating unit
- repeating units join until a large number of monomers have joined together
What are the molecules called that have had their double bond broken, before they form monomers?
Repeating units
How is a polymer represented?
The repeating unit, in brackets, with the small n
Examples of addition polymers?
DNA, silk, rubber
How is the repeating unit written?
Without brackets or n - just what is inside the brackets
How are polymers named?
Adding poly as a prefix to the name of the monomer
What is the polymer produced when ethene is the monomer?
Poly(ethene)
Are addition polymers saturated?
Yes
Why are addition polymers unreactive?
They are saturated
What do the intermolecular forces between polymer chains depend on?
The atoms within the polymer and how they are arranged
Are polymer chains polar or non-polar?
Non-polar
What intermolecular force are polymer chains held together by?
Van der Waal’s forces (unless electronegative atoms like C/F are present)
When will polymer chains not be held together by Van der Waal’s forces?
When there are electronegative atoms like C and F present
Properties of high density poly(ethene)?
- fewer branches
- stronger IMFs
- stronger polymers, more rigid
What might HDPE be used to produce?
Plastic bottles, bottle caps, water pipes
Properties of low density poly(ethene)?
- more branched chains
- weaker IMFs
- weaker polymers, more flexible
What might LDPE be used to produce?
Plastic bags, plastic wraps, cable insulation
Which has stronger intermolecular forces; HDPE or LDPE?
HDPE as there is less branching
What is poly(chloroethene) commonly known as?
PVC - polyvinyl chloride
Description of polyvinyl chloride?
Hard and brittle
Bonding in polyvinyl chloride?
Permanent dipole-dipole forces between molecules due to electronegativity of Cl in C-Cl bonds
What might polyvinyl chloride be used to produce?
Window frames, drain pipes
Has the knowledge and understanding of the production and properties of polymers changed over time?
Yes
What can be added to polymers to change their properties?
Plasticisers
What do plasticisers do?
Force chains of polymers further apart, allowing them to slide over each other, making them m`ore flexible
For a symmetrical alkene to undergo electrophilic addition with a halogen, how must the halogen be beforehand?
The halogen molecule dissolved in water or in an organic solvent
Conditions for addition polymerisation?
High pressure
