alkenes Flashcards
what are alkenes
alkenes are unsaturated hydrocarbon
they are made up of hydrogen and have one or more carbon carbon double bonds
what does the double bond in alkenes do to the number of hydrogens in alkenes
it mean that alkenes have fewer than maximum possible number of hydrogen atoms
what does the double bond do to the reactivity of alkenes
the double bond makes alkenes more reactive than alkanes because of the high concentration of electrons (high electron density)between the two carbon atoms
what are alkenes used for
ethene is a starting material for a large range of products
- the starting material for a range of products e.g.
polymers such as polyethene e.t.c - paints, antifreeze
what is the general formula for alkenes
CnH2n
what is the unique nature of alkenes
unlike the C-C bonds in alkanes, there is no rotation about the double bond
this is because of the make - up of a double. Any molecules in which a hydrogen atom in ethene is replaced by another atom or group will have the same flat shape around the carbon - carbon double bond
why can’t the double bond cannot rotate
as well as a normal C-C single bond, there is a p - orbital
(which contains a single electron)
these 2 orbitals overlap to form an orbital with a cloud of electron density above and below the single bond
This Is called a pi orbital and its presence means the bond cannot rotate.
This is sometimes called restricted rotation
what are the two types of isomers that involve the double bond
- position isomers
- geometrical isomers
what are the position isomers
they are isomers where the double bond is in different positions
the longer the carbon - carbon, the more possibilities there will be and therefore the greater the number of isomers
what are geometrical isomers
it is a form of stereoisomerism
the two stereoisomers have the same structural formula but the bonds are arranged differntly in space
it occurs only around the C=C bond
e.g. but-2-ene can exists as:
z-but-2-ene
E-but-2-ene
what does the E-Z rotation based off of
the E-Z rotation is based on atomic numbers
HOW DO WE DO THIS:
-look at the atoms attached to each of the carbon atoms in the double bond
when the two atoms (of each pair) of a higher atomic number are on the same side of the C=C, the isomer is described as Z
the other isomer has the position of the Cl and hydrogen reversed
This is described as an E isomer
what are the physical properties of alkenes
the double bond does not greatly affect properties such as boiling and melting points
- van der Waals forces are the only intermolecular forces that act between the alkene molecules
This means that thee physical properties of the alkenes are very similar to those of the alkenes - melting and boiling points increase with the number of carbon atoms present
- alkenes are not soluble in water
- are non - polar because the electronegativity of carbon and hydrogen are similar - hence why the intermolecular forces between their molecules are (weak) van der Waals forces
how do alkenes react
the double bond makes a big difference to the reactivity of the alkenes compared with alkanes
The bond enthalpy for C=C is 612kJ mol–1 so you might predict that alkenes would be less reactive than alkanes
HOWEVER, ALKENES ARE MORE REACTIVE THAN ALKENES
The C=C forms an electron-rich area in the molecules, which can easily be attacked by positively charged reagents are called electrophiles (electron liking)
what are some examples of good electrophiles
an example of a good electropile (electron liking)is the H+ ion
what are most reactions of alkenes
As alkenes are unsaturated they can undergo addition reactions
In conclusion, most of the reactions of alkenes are electrophiles
do alkenes burn in the air
alkenes will burn in the air
However, they are not used as fuels. This is because their reactivity makes them very useful for other purposes
what electrons are used in electrophilic substitution
reactions of alkenes ar typically electrophilic addition.
The four electrons in the carbon- carbon double bond make it a centre of high electron density
electrophiles can form a bond by using two of the four electrons in the carbon carbon double bond (of the four electrons, the two are in a pi bond)
what is the mechanism of electrophilic substitution
- 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 ion or have positively area
- a positive ion ( a carbocation) is formed
- a negatively charged ion forms a bond with the carbocation
how do alkenes react with hydrogen halides (an electrophile
hydrogen halides e.g. HCl , HBr and HI add across the double bond to form a haloalkane
e. g. imagine HBr reacting with alkenes
1. Bromine is more electronegative than hydrogen, so the hydrogen bromide molecule is polar H+ — Br-
2. the electrophile is the H+ of the H+of the HBr
3. One of the pairs of the electrons from C=C forms a bond with the H+ to form a positive ion (called a carbocation), whilst at the same time, the electrons in the HBr bond are drawn towards the BR-
4. The bond in hydrogen bromide breaks hetrolytically. Both electrons from the shared pair in the bond go to the bromine atom because it is more electronegative than hydrogen leaving a Br-ion
5. The Br-ion attaches to the carbocation forming a bond with one of its electron pairs
what happens when the double bond is not exactly in the middle
when hydrogen bromide adds to the ethene,bromoethane is the only possible product
However, when the double bond is not exactly in the middle of the chain, there are two possible products-the bromine of the hydrogen bromide could bond to either of the carbon atoms of the double bond
what are the two products of the electrophilic substitution of propene with HBr
it is 2-bromopropane / 1-bromopropane
however, the main product is entirely 2-bromopropane
why is 2- bromopropane the main product while
alkyl groups e.g. CH3 or C2H5 for example, have a tendency to release electrons
This is known as a positive inductive effect and is sometimes represented by an arrow along their bond to show the direction of the release
This electron releasing effect tends to stabilise the positive charge of the intermediate carbocation
Essentially, the more alkyl groups there are attached to the positively charged atom, the more stable the cation is
which alkanes are more stable tertiary, primary or secondary
so, a positively charged carbon atom which has three alkyl groups called a tertiary carbocation is more stable than one with two alkyl groups (secondary carbocation) which is more than one with just one (primary carbocation)
tertiary> secondary > primary
how are alkenes converted to alcohols
they are converted to alcohols in a two-step reaction
first with an electrophilic addition reaction with H2S04 and the hydrolysis with water
H2SO4 is regenerated by the ned of the reactions an so it acts as a catalyst
