6. Organic molecules (REACTIONS OF ALKENES)
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Give the general formula for alkenes
CnH2n
Are alkenes saturated or unsaturated
unsaturated
What does the C=C bond in an alkene cause
Restricted rotation due to pi bond
Why are alkenes insoluble
They cannot form hydrogen bonds with water. They only have london forces and permanent dipole dipole forces.
What are the 7 types of reactions alkenes can have
Addition Polymerisation Elimination Hydrolysis Oxidation Reduction
What does a curly arrow show
How electron pairs move around
What is an electrophile
a species attracted to areas of high electron density and a electron pair acceptor
Describe what happens when an a halogen reacts with an alkene during an electrophilic addition reaction
Forms a dihalogenoalkane As the halogen (eg: Br-Br) approaches the pi bond electrons repel the electron pair in bromines bond, inducing a dipole - becoming polar and electrophilic 2) The slightly positive bromine atom bonds to a carbon in the alkene 3) This leaves a negative Br ion and the intermediate carbocation with a positive charge 4) the Br ion therefore forms a bond with the positive charged carbocation
Conditions needed for a dihalogenoalkane reaction
Room temperature and pressure
What must you remember to do when drawing an electrophilic mechanism
Draw the dipoles ( slight positive closer to C=C bond) draw a curly arrow from the C=C bond to the slight positive then another curly arrow from the middle of the dipole bond to the slight negative In the intermediate stage, draw carbocation (C+) and have a curly arrow from lone pair of electrons to the carbocation and make sure to include charge of the element (note it is not a slight anymore)
Describe what happens in hydrogenation and give the required conditions
An alkene reacts with hydrogen forming an alkane Conditions: –> Nickel catalyst –> 150 temperature –> Hydrogen (we produce margarine in this way)
How is the major product formed
Forms a halogenoalkane If unsymmetrical, major and minor product formed. Major product is formed via the most stable carbocation which is the tertiary carbocation. Basically for the major product, add the hydrogen to the carbon with the most hydrogens already attached. This is Markovnikov’s rule
List in increasing stability, the type of carbocation
primary - 1 R goup secondary - 2 R group tertiary - 3 R group ( alkyl groups are electron releasing and so reduce the charge on the ion, stabilising it)
Describe the reaction of alkenes with potassium manganate
• shake alkene with acidified solution room temp • purple colour becomes colourless due to oxidation reaction which forms a diol another test for saturation
Describe the reaction of alkenes with bromine water and observation
turns from orange to colourless
Describe the reaction of alkenes and water/steam (hydration)
forms an alcohol - high temp, high pressure - concentrated phosphoric acid catalyst (H3PO4) - steam no waste therefore high atom economy
Give an equation to show the major product for the reaction between ethene with both bromine WATER and bromine LIQUID
With bromine water: C2H4 + Br2 (aq)+ H2O —> CH2(OH)CH2Br + HBr With bromine liquid: C2H4 + Br2 (l) —> CH2BrCH2Br
Explain how the halogen acts as an electrophile in electrophillic addition with alkenes
As the X2 approaches the C=C bond, the electrons experience repulsion and are pushed away from the electron dense C=C. An induced dipole with delta + close to C=C bind and delta - further away from C=C bond is formed