Alkenes Flashcards
Sigma bond
Consists of 2 overlapping s orbitals (covalent bond) - strong attraction between positive nucleus and electron cloud makes sigma bonds very strong
Pi bond
Consists of 2 overlapping p orbitals (2nd covalent bond in a C=C bond) - Has no electrons in the middle but electron cloud is in the top and bottom section - this makes pi bonds weaker than sigma bonds.
What type of reactions do alkenes undergo and what happens to the bonds in these reactions.
Alkenes undergo addition reactions where (in the C=C bond) the pi bond breaks but the sigma bond remains intact. In these addition reactions, electrophiles are attracted towards the double bond, which contains a region of high electron density.
Describe how boiling points of alkenes change as chain length increases
As chain length increases, Surface area increases and van der walls forces become more extensive.
What is an addition reactions
A reactions where 2 molecules join together to form a larger molecule - one of these must have a double bond.
Explain why H+ , Br-Br,H-Br are electrophiles
Test for alkenes
Add bromine water to alkene - bromoalkane is formed and solution goes from orange / brown to colourless
Electrophilic addition with bromine mechanism
Electrophilic addition with hydrogen halides mechanism
Unsymmetrical alkenes
When a compound HX is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon with the most hydrogens attached to it already.
Explanation of markovnikov’s rule
Alkyl groups (R) are electron releasing. This means they partially release the electrons in a bond with an atom - onto the other atom making it more -ve / less +ve.
Primary carbocations contain a positively charged carbon which is bonded to one alkyl group
Secondary carbocations contain a positively charged carbon which is bonded to two alkyl group
Tertiary carbocations contain a positively charged carbon which is bonded to three alkyl group.
More alkyl groups attached to the positively charged carbon makes this charge less positive and more stable.
Tertiary carbocations are more stable than Secondary which are more stable than primary.
This means that the negative ion is attracted to the higher degree carbon, and the positive ion is attracted to the lower degree carbon (the one with more hydrogens). This explains markovnikov’s rule.
Electrophilic addition with concentrated sulphuric acid mechanism
Alcohol production
Conditions:
60 atm / 6000kPa
300 C
Concentrated H2SO4 catalyst
Electrophilic addition with water mechanism
Conditions:
catalyst of phosphoric acid - H3PO4
300 C
70 atm pressure
Produces alcohols.
Explain how alkenes can undergo polymerisation reactions to form a polymer chain.
Pi bond breaks and the e- is used to form a bond with the C atom on the neighbouring molecule.
Conditions required to bring about polymerisation
A Ziegler - Natta catalyst
Uses of polyethene
Plastic bags
Uses of polypropene
Carpets ropes and crates
Ways of disposing polymers
Markovnikov’s rule
When a compound HX is attached to an
