Liquid bromine is prepared by dissolving Br2 in carbon tetrachloride (CCl4). Br2 is a non-polar molecule and they dissolve in non-polar solvents such as carbon tetrachloride. Liquid bromine in carbon tetrachloride has red-brown colour.
If one double exists in the alkene molecule, it reacts with one molecule of Br2 and two bromine atoms are added to the alkene to form alkyl halide compounds.
This reaction proves, alkenes are unsaturated organic compounds.
This reaction is the reaction between alkenes and halogens. In this tutorial, we are going to discuss about alkenes reactions with bromine liquid, and mechanisms of these reactions. We learn ethene and bromine reaction and propene and bromine reaction.
When Br2(l) is mixed with alkene, orange / brown colour of bromine liquid is reduced.
Here, we are going to discuss about some reactions and products of liquid bromine and alkenes such as ethene, propene, 2-butene and more. Mechanism of ethene and propene with Br2(l) are explained also.
Ethene is a symmetric alkene molecule. Ethene reacts with liquid bromine to give 1,2-dibromoethane. Ethene and bromine reaction also gives a symmetrical alkyl halide compound. Two bromine atoms are attached to the two carbon atoms in the ethene molecule. This reaction can be done at room temperature or below temperatures than room temperature.
Addition of bromine to ethene is an electrophilic addition reaction. Normally, bromine molecule (Br2) is not electron deficiency compound. But due to higher electrons density in the double bond of ethene, bromine molecule gets polarized. The most closed atom of bromine molecule to the double bond is positively charged (Brδ+) and other bromine atom gets negatively charged (Brδ-).
The positively charged bromine atom (electron deficiency atom) is attacked by the double bond and make bonds with both carbon atoms of double bond. Other bromine molecule forms a bromide ion.
A cyclic brominium ion is given as the intermediate product. Next, one carbon atom in the brominium ion is attacked by bromide ion (Br-). Then a bond between Br- and that carbon atom is formed and existing bromine - carbon bond in the ring is broken. A dibromide compound is formed as the only product. No minor products are given in this reaction.
Brdelta;+ is an electrophile and it starts the reaction. Also it adds to through the pi bond. Hence we call this reaction is an electrophilic addition reaction.
Propene reacts with bromine and add two bromine atoms give the 1,2-dibromopropane. Two bromine atoms are added to the double bond carbon. Ethene was a non polar alkene. But propene is a polar molecule. This reaction is also an electrophilic addition reaction.
As bromine addition to ethene, propene and bromine reaction occurs in same way.
First, Br2 molecule is polarized by alkene bond. With that, electrons in the double bond attacks the positively charged Br atom and forms a cyclic bromonium ion. Other Bromine atom is eliminated as a bromide ion.
As second step, produced bromide ion attacks a carbon atom which has a bond with other bromine atom. Then one of previous carbon - bromine bond is broken and a new carbon - bromine bond forms.
In this case, we have to go to the mechanism of the reaction to understand what will happen and what are the products can be given.
Halide ions can be given from as NaCl or NaI.
In the reaction, Br+ is the only cation and it is connected to the two carbon atoms which had the double bond. So it forms cyclic brominium ion.
In next step, Br- or Cl- or I- can connect to give three different products if both Cl- and I- are in the presence with Br2.
Following product may be given.
According to the anion concentration, major product is given by higher concentrated anion.
Ethene aand ethyne are alkene and alkyne respectively. Alkynes also react with bromine liquid and give alkyl halides as products. But difference is alkyne can reacts with two bromine molecules while alkene is limited to one bromine molecule if these is only one double bond in the alkene. So four bromine atoms will be added to the ethyne molecule when excess bromine liquid is added. Otherwise only ttwo bromine atoms will br added.
1,2-dibromopropane is the IUPAC name of the product which is an alkyl dihalide compound.
The product, alkyl dihalide are used to produce alkyne. 1,2-dibromoethane is treated with alcoholic KOH to prepare ethyne.
You want to prepare 1,2-dibromoethane from ethene. But you do not have liquid bromine to do this conversion. Then, suggest a method this can be done.
Ethene reacts with cold alkali KMnO4 (Baeyer's reagent). It gives ethane-1,2-diol.
Next ethane-1,2-diol reacts with PBr3 or HBr to produce 1,2-dibromoethane.
Due to addition of two bromine atoms through double bond, this is an addition reaction. And Brδ+ which is an electrophile starts the reaction. So this reaction is an electrophilic addition reaction.
An electrophilic addition reaction occurs. First, a bromine atom is added to the propene through double bond. Due to predence of water, there are hydroxyl ions with bromide ion. So in the seconsd step, hydroxyl ions can be added to cyclic brominium ion like bromide ions.
Here we are going to identify benzene and alkene from the comparing of reaction with bromine.
Benzene does not react with Br22 / CCl4. Therefore brown/ orange colour is not changed with time.
But with alkene, orange colour of bromine is reduced due to the reaction.
According to the colour difference, benzene and alkene can be identified.
As alkene, alkynes also react with bromine liquid. When there is excess bromine, four bromine atoms are added to the alkyne. So, ethyne reacts with bromine liquid to produce tetrabromoethane.