Organic Chemistry Help Please? D:

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2 Answers
Feb 6, 2018

#Na_2Cr_2O_7 + H_2SO_4 + H_2O# under reflux.


This is an oxidation reaction. Adding oxygen means oxidation, adding hydrogen would be a reduction reaction. If you look at the structure of toluene below on the left and benzoic acid on the right, you can see that we need to add in two oxygen atoms.

#LiAlH_4# and #H_2# are sources of hydrogen so they are not going to add any oxygen. NBS is a source of bromine so it won't help.

So you need an oxidant, something with a source of oxygen. The #CO_2# can be used to carboxylate certain things, it acts as an electrophile, so you would need a nucleophile (such as the pi electrons in benzene) to attack it. This would lead to a substituted toluene but would not replace the methyl group.

In this case a strong oxidising agent such as acidified #Na_2Cr_2O_7# under reflux (need heat) will oxidise an alkyl side chain on a benzene ring to a carboxylic acid. You could also use another strong oxidising agent instead, such as #KMnO_4#.

An interesting thing to note is that toluene is not miscible in water. But it is slightly soluble and possibly more soluble in acidic solution.

Feb 8, 2018

We could write separate redox reactions....


Dichromate is reduced to #Cr^(3+)#...

#Cr_2O_7^(2-)+14H^+ + 6e^(-) rarr 2Cr^(3+) + 7H_2O(l)#

And toluene is oxidized to benzoic acid...

#C_6H_5stackrel(-III)CH_3 +2H_2Orarr C_6H_5stackrel(+III)CO_2H+ 6H^+ +6e^(-)#

And we add the equation together to eliminate the electrons...

#Cr_2O_7^(2-)+8H^+ +C_6H_5CH_3 rarr C_6H_5CO_2H +2Cr^(3+) + 5H_2O(l)#

So we use the acidified sodium dichromate...and we observe a change in colour from the orange-red of dichromate to the green chromic ion...