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
Haloalkanes react with ammonia to form….
Amines
What type of reaction takes place when haloalkanes react with ammonia dissolved in ethanol and form amines
Nucleophiles substitution
Haloalkanes react with …….. to form nitriles
Cyanide
What type of reaction takes place when a warmed haloalkane reacts with potassium cyanide to form nitriles
Nucleophilic substitution
What is the product of an elimination reaction
An alkene
Explain elimination reaction between haloalkanes and hydroxide ions
The warmed haloalkane is added to concentrated potassium hydroxide or sodium hydroxide in concentrated ethanol solution and an alkene is formed
Why must the mixture be refluxed in an elimination reaction
So you don’t lose the volatile components of the mixture
Conditions in elimination reaction
Reflux
Anhydrous
What happens in an elimination reaction
A small group of atoms breaks away from a larger molecule. This small group is not replaced by anything else (whereas it would be in a substitution reaction)
The type of reaction depends on the……
Conditions
What conditions result in nucleophilic substitution
Aqueous
OH- acts as nucleophile
Reflux
What conditions result in an elimination reaction
Anhydrous
OH- acts as base
Reflux
Advantage of elimination reaction
Good way of getting double bond in molecule. Loads of other organic synthesis reactions use alkenes, so this reaction is good starting point for making lots of different organic chemicals
Advantage of nucleophilic substitution reaction
Allows you to produce any alcohol molecule needed. Starting point for synthesis reactions that produce aldehydes, ketones, esters carboxylic acids
Alkenes are what type of hydrocarbons
Unsaturated
Why are molecules with a double bond unsaturated
Because they can make more bonds with extra atoms in addition reactions
Why does the C=C bond have a high electron density
Because there is 2 pairs of electrons within it, making the alkene reactive.
What happens in an electrophilic addition reaction
The two pairs of electrons in the C=C bond means so high electron density. The double bonds open up and atoms are added to the carbon atoms.
Electrophilic addition reactions happen because the double bond has got plenty of electrons and is easily attacked by Electrophiles. The double bond is also nucleophilic- it’s attracted to places that don’t have enough electrons
What are Electrophiles
An ion or molecule that is able to accept an electron pair as they don’t themselves have enough electrons. They are attracted to areas with lots of electron pairs.
Examples of Electrophiles
Positively charged ions (H+)
Polar molecules (the electropositive atom is attracted to places with lots of electrons
Test for bromine water
Bromine water, turns from orange to colourless (through electrophilic addition)
What is a carbocation
An organic ion containing a positively charged carbon atom
Which type of haloalkanes are hydrolysed the fastest
Iodoalkanes
Iodoalkanes are the fastest at what? Explain
They are hydrolysed the fastest
The carbon halogen bond strength decides reactivity, for a reaction to occur the carbon halogen bonds need to break. The C-F bond is the strongest as it has the highest bond enthalpy. So fluoroalkanes hydrolysed the slowest. The C-I is the weakest as it has the lowest bond enthalpy making it easier to break so they are hydrolysed the fastest
Alkenes undergo addition with hydrogen bromide to form…..
Bromo alkanes
Alkenes can undergo electrophilic addition with three compounds…what are these
Hydrogen bromide
Halogen
Water and sulphuric acid
Adding hydrogen bromide to unsymmetrical alkenes forms how many products
2
The amount of each product depends on how stable the carbocation formed in the middle of the reaction is. Carbocations with more alkyl groups are more stable because the alkyl groups feed electrons towards the positive charge. The more stable carbocation is more likely to form
What are alkyl groups
Alkanes with a hydrogen removed eg methyl
Reacting alkenes with water and a sulphuric acid catalyst makes….
Alcohols
How are alcohols produced industrially
By hydrating alkenes in the presence of an acid catalyst such as sulphuric acid
Explain the process of hydrating alkenes in the presence of an acid catalyst
Cold concentrated sulphuric acid reacts with an alkene in an electrophilic addition reaction
Add cold water and heat, the product is hydrolysed to form an alcohol
The sulphuric acid is reformed at the end. Shows that it works as a catalyst
Ethanol is manufactured by …..
Steam hydration
How is ethanol manufactured by steam hydration
Ethene hydrated by steam at 300c, 60atm. Needs solid phosphoric acid catalyst. Reaction is reversible and reaction yield is low, only about 5%. Recycle the unreacted ethene gas, making the overall yield about 95%
E/Z isomerism so a type of….
Stereoisomerism
What are stereoisomers
They have the same structural formula but a different arrangement in space.
Explain how E/Z isomers can form
Due to the lack of rotation around the double bond, some alkenes can have stereoisomers. These happen when the 2 double bonded carbon atoms each have different atoms or groups attached to them.
When do stereoisomers happen
When the 2 double bonded carbon atoms each have different atoms or groups attached to them
Alkenes join up to form….
Addition polymers. The double bonds in alkenes can open up and join together to make long chains called polymers. The small individual alkenes are called monomers. Called addition polymerisation
What is addition polymerisation
When the double bonds in alkenes open up and join together to make long chains called polymers
Properties of addition polymers
Due to the loss of the double bond, poly(alkenes) are unreactive, like alkanes. Different polymer structures have different properties which means they are suited to different uses. Many polymers are difficult to dispose of and are made from non renewable oil fractions so they are recycled. Eg poly(propene) is recycled, it can be melted and remoulded
Give a use for poly(ethene)
Plastic bags
Give a use for poly(propene)
Rope
