More organics UNIT 2 Flashcards
Why can’t a double bond rotate
Aswell as a normal C-C single bond,there is a p orbital on each carbon. These two orbitals overlap, as all the atoms lie in the same plane making the alkene trigonal planar in shape, to form one single orbital with a cloud of electron density above and below the single bond. This is called a pi orbital and it’s presence means the bond can not rotate.this is sometimes called restricted rotation and this is what causes E/Z isomerism
What is an alkene
They are unsaturated hydrocarbons made from carbon and hydrogen atoms only and have one or more C-C double bonds. This means that alkenes have fewer than the maximum possible number of hydrogen atoms. The double bond makes them more reactive than alkanes. Alkenes are produced in large quantities when crude oil is thermally cracked
Physical properties of alkenes
The double bond doesn’t really affect the properties such as boiling and melting points. Vdw forces are the only intermolecular forces that act between the alkene molecules. This means that the physical properties of alkenes are very similar to those of the alkanes. The melting and boiling points increase with the number of carbon atoms present. Alkenes are not soluble in water because they are non polar
Are alkenes more or less reactive than alkanes? Why?
More reactive
The C-C double bond forms an electron rich area in the molecule, which can easily be attacked by positively charge reagents. These reagents are called electrophiles. As alkenes are unsaturated they can undergo addition reactions ( mainly electrophilic addition)
What is the test for a double bond
Bromine solution/bromine water added to alkene. The solution goes from brown to colourless
Electrophilic addition mechanism
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 a positively charged area
A positive ion (carbocation) is formed
A negatively charged ion forms a bond with the carbocation
Hydrogen halides mechanism
With HBr…
Bromine is more electronegative than hydrogen, so the hydrogen bromide molecule is polar
The electrophile is the slightly positive hydrogen
The slightly positive hydrogen of the HBr is attracted to the C-C double bond because of the double bonds high electron density
One of the pairs of electrons from the C-C double bond forms a bond with the slightly positive hydrogen to form a positive ion (carbocation) while at the same time the electrons in the HBr bond are drawn towards the slightly negative bromine
The bond in hydrogen bromide breaks heterolytically. Both electrons from the shared pair in the bond goes to the Br atom because it is more electronegative than H to leave a Br- ion.
This ion attaches to the positively charged carbon of the carbocation forming a bond with one of its electron pairs
Why don’t alkanes react with most chemicals
The C-C and C-H bonds within the alkanes are non polar. So this means that alkanes don’t react with polar chemicals as they don’t have an polar groups. Alkanes will react with non polar chemicals eg oxygen or the halogens , but only if they are provided with enough energy
Halogens react with alkanes to form…
Haloalkanes
What type of reactions do alkanes and halogens undergo to form haloalkanes
Photochemical reactions
Photochemical reactions are started by…..
UV light
What is free radical substitution
When alkanes and halogens react together in a photochemical reaction. This is started by UV light. A hydrogen atom is substituted by chlorine or bromine
When are free radicals produced? What are their properties
when bonds split equally
Highly reactive
Chloroalkanes and chlorofluoroalkanes are used as…
Solvents
CFC’s (carbonfluorocarbons) are haloalkene molecules where all the hydrogen atoms have been replaced by chlorine and fluorine atoms
Chlorine atoms are destroying the….
Ozone layer
Ozone in the upper atmosphere acts as a chemical barrier. It absorbs UV radiation which can cause sunburn and skin cancer
Where is ozone found
In the upper atmosphere
What is the role of ozone
It acts as a chemical barrier, by absorbing UV radiation
What does ozone prevent
UV radiation causing sunburn and skin cancer
How is ozone formed
Naturally when an oxygen molecule is broken down into 2 free radicals by UV radiation. The free radicals attack other oxygen molecules forming ozone
How is ozone being destroyed by CFC’s
The chlorine free radicals are formed when the C-Cl bonds in CFC’s are broken down by UV radiation. The free radicals are catalysts. They react with ozone to form an intermediate and oxygen molecule
CFC’s are now…..
Banned.
CFC’s are pretty unreactive, non flammable and non toxic. Were used in fire extinguishers, as coolant gas in fridges etc. In 1970’s scientists discovered that they were causing damage to the ozone layer. The disadvantages outweighed the advantages and the were banned. Chemists developed alternatives, HCFC’s (hydrochloroflurocarbons) and HCF’s (hydrofluorocarbons) which are less dangerous so are being used as temporary alternatives until safer products are developed
Fridges use ammonia or hydrocarbons as the coolant gas nowadays
What are haloalkanes
An alkane with at least one halogen atom in place of a hydrogen atom
The carbon halogen bond in haloalkanes is……
Polar
Halogens more electronegative than carbon. The electropositive carbon doesn’t have enough electrons. This means it can be attacked by a nucleophile
Examples of nucleophiles
OH-
CN-
NH3
Haloalkanes can be hydrolysed to make…..
Alcohols
What is needed to convert a haloalkane into an alcohol
Warm aqueous sodium or potassium hydroxide
What is the name of the reaction that hydrolyses haloalkanes into alcohols using warm acidified sodium or potassium hydroxide
Nucleophilic substitution