Alkenes Flashcards
What are alkenes?
- Alkenes are unsaturated hydrocarbons.
Why are alkenes fairly reactive?
- Alkenes have double bonds which have a HIGH ELECTRON DENSITY, making alkenes fairly reactive.
What are the double bonds in alkenes described as the centre of?
- Double bonds in alkenes = centre of high electron density.
How many hydrogens do cycloALKENES have compared to their straight-chain alkene counterparts?
- 2 fewer hydrogens than straight-chain alkene counterparts.
What is an “electrophile?”
- An electrophile is an electron-defficient species which can accept a pair of electrons, forming a dative covalent bond.
What attacks alkenes in addition reactions?
- Electrophiles.
- The electrophiles are then added to the molecule.
Give examples of electrophiles.
- Positively-charged ions (ie. H⁺)
- Polar molecules (with S+ charge): ie. HBr/ H₂SO₄.
What is the test for alkenes/ unsaturation? What acts as an elecrophile in this addition reaction/ what actually happens?
- Add bromine water to sample to test for unsaturation.
- If alkene is present, colour change = from brown-orange to colourless.
- Br₂ acts as electrophile.
- Br₂ is added to alkene to form dibromoalkane = colourless.
How does Br₂ become polarised as it approaches the C=C double bond in addition reaction of an alkene?
- Br₂ is polarised as the molecule comes near to C=C double bond as the electrons in the C=C double bond will repel the electrons in bond in Br- Br to one side of molecule, induced dipoles formed S+/ S-
Outline the mechanism of addition reaction of ethene (as example: could be any alkene!) using Br₂
Diagram for this (which you need to know!) is in the pink hand folder!!
- Br₂ is polarised as electrons in double bond will repel the electrons in Br₂.
- Pair of electrons in C=C double bond move to to S+ in bromine and forms a bond between C- Br.
- Pair of electrons move from Br-Br bond to Br, breaking Br- Br bond.
- Carbocation intermediate is formed, Br- is attracted to C+. Pair of electrons move from Br- to C+, bond is formed between C and other Br.
- Colourless 1,2-dibromoethane is formed.
Same concept for the hydrogen halides!
What is the intermediate formed in additon reaction of alkenes using Br₂ (OR any other electrophile) called?
- Carbocation intermediate.
What 2 different types of alkenes do we have? What happens when we react hydrogen halides/ sulphuric acid with them?
- Unsymetrical: 2 different products are formed.
- Symmetrical: only one product is formed.
If you react an unsymmetrical alkene with a hydrogen halide, what determines the AMOUNT of the two products that are formed ie. major vs minor products? How? Why?
- Determined by stability of carbocation intermediate.
- More alklyl groups DIRECTLY bonded to C+, more stable the intermediate is.
- This is because alkly groups will push electrons towards positive carbocation, stabilising it.
- More alkly groups = greater positive inductive effect of alkyl groups = carbocation more stable.
- More stable carbocations = more likely to form.
DRAWING OF THIS IN PURPLE SCHL CHEM FOLDER.
Defs of primary, secondary, tertiary carbocation intermediates?
- Primary: one alkly group directly attached to C+.
- Secondary: two alkyl groups directly attached to C+.
- Tertiary: three alkyl groups directly attached to C+.
DRAWING OF THIS IN PURPLE SCHL CHEM FOLDER.
Which carbocation intermediate is your MOST major product
primary, secondary or tertiary?
- Tertiary because it is the most stable so most likely to be formed, has greatest positive inductive effect.
Why is it that two products can be formed when you react alkene with hydrogen halide?
- Because, after C=C bond breaks, H can be added to one of 2 carbons, so, your halogen can end up bonded to one of two carbons.
DRAWING OF THIS IN PURPLE SCHL CHEM FOLDER.
When you react alkene with hydrogen halide, you will form two products. What is the name for these two products, what do they entail?
- Minor: less stable
- Major: most stable
DRAWING OF THIS IN PURPLE SCHL CHEM FOLDER.
What are the conditions needed for alkenes to react with sulphuric acid? What is product of reaction?
- COLD and CONCENTRATED sulphuric acid.
- Alkyl hydrogen sulphates.
What does the sulphuric acid act as when you make an alcohol from an alkene (2 step process!) How do we know it acts like this?
- Acts as catalyst.
- It is regenerated at the end of the 2-step process.
Other than H2SO4, how else can sulphuric acid be written?
H (S+) — O (S-) SO₃H
Need to make sure S- goes on top of the OXYGEN, not at the end by the H on OSO3H!!
How can you form an alcohol from alkyl hydrogen sulphates. What is this process called?
- Add cold water to warm alkyl hydrogen sulphate= alcohol = formed.
- Process = called hydrolysis.
If we start with an unsymmetrical alkene reacting with sulphuric acid, how many different types of alcohols can we form?
- We can form 2 alcohols!
Overall equations for 2-step process of forming alcohol from alkene. What is this two-step process often referred to as?
Alkene + sulphuric acid –> alkyl hydrogen sulphate
Alkyl hydrogen sulphate + water –> Alcohol + sulphuric acid
Referred to as: acid-catalysed hydrolysis.
What is the mechanism called when an alkene reacts with hydrogen halide/ sulphuric acid/ bromine?
- Electrophilc addition.
