Chapter 14 - Alkenes Flashcards
What is the shape and bond angles in an alkene
Alkenes are planar molecules with bond angles of roughly 120
What properties does the double bond in an alkene contain
It is a double covalent bond that is electron rich and cannot rotate.
why can’t a double bond in an alkene rotate
There are p orbitals on both carbon atoms with overlap to form a pi orbital meaning that the double bond cannot rotate (restricted rotation)
what two types of isomers are present in alkenes
position isomers
Geometrical isomers
what are position isomers
when the double bond is in different positions in the carbon chain. e.g. but-1-ene and but-2-ene
What are geometrical isomers
geometrical isomers are a form of stereoisomers with the same molecular formula but different structural formulas as the bonds are arranged differently.
what are the rules for E-Z isomerism
If the two atoms with the greatest atomic number are on the same side of the double bond they are Z isomers. If they are on opposite sides they are E isomers.
what are some physical properties of alkenes
The only intermolecular forces are Van der Walls. The double bond does not greatly affect the boiling point so the properties are similar to alkanes. Alkene are not soluble in water.
Are alkenes more or less reactive than alkanes
Although the bond enthalpy is higher in alkenes they are more reactive due to electrophiles
What are electrophiles with examples
electron pair acceptors
halogens
hydrogen halides
sulfuric acid
water (phosphoric acid catalyst)
picture of bonding in an alkene
example of position isomers in butene (picture)
example of geometrical isomers in butene (picture)
what are the steps in elecrophilic addition
- The electrophile is attracted to the double bond
- electrophiles are positively charged and accept a pair of electrons from the double bond. The electrophile may be a positively charged area.
- a positive ion is formed (carbocation)
- a negeatively charged ion forms a bond with the carbocation
what is hetrolytic bond breaking
when a cocalent bond breaks both electrons go the same atom and none of them to the other atom
outline the mechanism between ethen and hydrogen bromide (HBr)
what is the positive induvtive effect in carbocations
When an atom or group contributes electrons to form a bond and gains a partial positive charge. This means that the more alkyl groups attached to a carbon the more neutral its partial charge meaning it is more stable which is why a tertiary carbocation is more stable than a primary carbocation.
what the the trend in stability between primary and tertiary carbocaation
outline the mechanism between ethene and bromine (Br2)
what is the test for a double bond
add bromine waater - orange brown –> colourless
outline the mechanism of the electrophilic addition between sulfuric acid aand ethene
what is the reaction between ethene and water when water acts as an electrophile and what are the conditions for the reaction
phosphoric acid catalyst
what is the equation for the polymerisation of ethene
what are some examples of natural polymers
proteins, cellulose, DNA
examples of polymers with their uses
what are addition polymers
Addition polymers are formed by the addition of some simple monomer units repeatedly.
what are plasticisers
plasticisers are additives that get between polymer chains allowing them to slide across eachother modifying their properties making them more felxible
