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A reaction is shown. On the left appears an alcohol and on the right, a carbonyl group. Above the reaction arrow appears the word “oxidation.” The alcohol is represented as a C atom with dashes to the left and below, an H atom bonded above, and an O atom bonded to an H atom in red connected to the right. The O atom has two sets of electron dots. The carbonyl group is indicated in red with a C atom to which an O atom is double bonded above. Dashes appear left and right of the C atom in black. The O atom has two sets of electron dots.

Oxidation and reduction in organic chemistry

Methane represents the completely reduced form of an organic molecule that contains one carbon atom. Sequentially replacing each of the carbon-hydrogen bonds with a carbon-oxygen bond would lead to an alcohol, then an aldehyde, then a carboxylic acid (discussed later), and, finally, carbon dioxide:

CH 4 CH 3 OH CH 2 O HCO 2 H CO 2

What are the oxidation numbers for the carbon atoms in the molecules shown here?

Solution

In this example, we can calculate the oxidation number (review the chapter on oxidation-reduction reactions if necessary) for the carbon atom in each case (note how this would become difficult for larger molecules with additional carbon atoms and hydrogen atoms, which is why organic chemists use the definition dealing with replacing C–H bonds with C–O bonds described). For CH 4 , the carbon atom carries a –4 oxidation number (the hydrogen atoms are assigned oxidation numbers of +1 and the carbon atom balances that by having an oxidation number of –4). For the alcohol (in this case, methanol), the carbon atom has an oxidation number of –2 (the oxygen atom is assigned –2, the four hydrogen atoms each are assigned +1, and the carbon atom balances the sum by having an oxidation number of –2; note that compared to the carbon atom in CH 4 , this carbon atom has lost two electrons so it was oxidized); for the aldehyde, the carbon atom’s oxidation number is 0 (–2 for the oxygen atom and +1 for each hydrogen atom already balances to 0, so the oxidation number for the carbon atom is 0); for the carboxylic acid, the carbon atom’s oxidation number is +2 (two oxygen atoms each at –2 and two hydrogen atoms at +1); and for carbon dioxide, the carbon atom’s oxidation number is +4 (here, the carbon atom needs to balance the –4 sum from the two oxygen atoms).

Check your learning

Indicate whether the marked carbon atoms in the three molecules here are oxidized or reduced relative to the marked carbon atom in ethanol:

A molecular structure is shown. A C H subscript 3 group is bonded up and to the right to a C H subscript 2 group. Bonded to the C H subscript 2 group down and to the right is an O H group.

There is no need to calculate oxidation states in this case; instead, just compare the types of atoms bonded to the marked carbon atoms:

Three molecular structures are shown, each with a red central C atom. In a, a C H subscript 3 group is bonded to the lower left, an H atom is bonded above, and H subscript 2 appears to the right of the central C atom. In b, an O atom is double bonded above the central C atom, a C H subscript 3 group is bonded to the lower left, and an H atom is bonded to the lower right. In c, an O atom is double bonded above the central C atom, a C H subscript 3 group is bonded to the lower left, and an O H group is bonded to the lower right.

Answer:

(a) reduced (bond to oxygen atom replaced by bond to hydrogen atom); (b) oxidized (one bond to hydrogen atom replaced by one bond to oxygen atom); (c) oxidized (2 bonds to hydrogen atoms have been replaced by bonds to an oxygen atom)

Aldehydes are commonly prepared by the oxidation of alcohols whose –OH functional group is located on the carbon atom at the end of the chain of carbon atoms in the alcohol:

A reaction is shown. An alcohol appears on the left and an aldehyde on the right of the reaction arrow. The alcohol is shown as C H subscript 3 C H subscript 2 C H subscript 2 O H, and the aldehyde is shown as C H subscript 3 C H subscript 2 C H O. The O H group at the right end of the alcohol structure and the C H O group at the right end of the aldehyde structure are in red.

Alcohols that have their –OH groups in the middle of the chain are necessary to synthesize a ketone, which requires the carbonyl group to be bonded to two other carbon atoms:

A reaction is shown. An alcohol appears on the left and a ketone on the right of the reaction arrow. The alcohol is shown as C H subscript 3 C H ( O H ) C H subscript 3 and the ketone is shown as C H subscript 3 C O C H subscript 3. The O H group in the alcohol structure and the C O group at the center of the ketone structure are in red.

An alcohol with its –OH group bonded to a carbon atom that is bonded to no or one other carbon atom will form an aldehyde. An alcohol with its –OH group attached to two other carbon atoms will form a ketone. If three carbons are attached to the carbon bonded to the –OH, the molecule will not have a C–H bond to be replaced, so it will not be susceptible to oxidation.

Questions & Answers

what is the periodic table all about
Godstime Reply
am new here looking forward on learning new stuff
Godstime
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Godstime
yeah
samuel
it's about predicting the different types of matter around us and organizing them in an elegant way
William
what is boyle law
Daniel Reply
the law states that at a constant absolute temperature the volume of a fixed of a gas is inversely proportional to the pressure of the gas...
Johnson
write short not on paper chromAtogphy
Ujunwa Reply
I am sorry, I don't understand the question. Are you referring to photospectometry, a machine that uses lightwaves to determine the chemical composition of a substance? Instead of the "coffee filter and solvent" method which is more crude, but simple enough to understand basic concepts.
Melissa
kkk
samuel
what is esterification?
Darey Reply
Esterification is the general name for a chemical reaction in which two reactants (typically an alcohol and an acid) form an ester(-COOR) as the reaction product....
Johnson
pls i dont know chemistry wat shall i do?
Chancha
Please...what do you mean by you don't know chemistry..can you explain further
Johnson
I am afraid you may end up injuring yourself working with these chemical processes with limited lab experience. Please be extremely careful. Research chem analysis, precedure and equipment.
Melissa
Kkk
samuel
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samuel
how we Learn full periodic table
muhammad Reply
how
Imran
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Angel Reply
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Michael
explain the atomic orbital
Masti Reply
2nacl➡️2na+cl2 2.58.5g 22.4l
suvarthavani Reply
what is neclear energy
Moyosoreoluwa Reply
nuclear energy is the energy released during nuclear fission or fusion.especially when used to generate electricity
Inusah
How many processes to break down CARBON DIOXIDE into carbon and oxygen
Prabhakar Reply
biology is my best subject
Oseni Reply
o really
Ajila
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Izadora
You're in the wrong place lmao
Zeus
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Charles
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daniel
hy
Kareem
How are you guys
daniel
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Kareem
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Izadora
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Kareem
Brazil
Izadora
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Kareem
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Izadora
Am from malawi
Hele
I'm from Nigeria, my name is Vincent. what is the relationship between volume an temperature?
Vincent
who is the father of chemistry?
Vincent
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Vincent
fine
Kareem
the relationship between volume and temperature is. volume is directly proportional to temperature
Inusah
Antoine lavoisier
Ujunwa
good evening everyone my name is Maryann
Ujunwa
an acid is any substance which when dissolved in water produces hydrogen ion as its only positive ion
Ezemadu Reply
its just me or you guys also hate chemistry?
Izadora
I partially hate it
Oseni
there's some parts that I actually like to study
Izadora
sometimes
Izadora
tell me about it
Oseni
like radiation
Izadora
physics have this too
Izadora
good evening everyone my name is Maryann
Ujunwa
Why acid is sour in taste. Please don't say naturally because there are a scientific reason in all natural thing
Prabhakar Reply
since it has a negatively charge nitrogen ions
Elvis
sorry, positively hydrogen ions, which has a pH less than 7
Elvis
Corrct, as the H+ ion concentration increases acidity increases.
Aakash
because it's pH value is smaller than 7.
Raju
what is molecules
samuel
what is acid
Faree
How we make negative mass
Prabhakar Reply
How we make negative and positive matters
Prabhakar Reply
matters are neutral in general.
Rakesh
It becomes positive or negative when they either loss or gain electrones .
Rakesh
Have you any doubt regarding the first question that you had asked.
Rakesh
It also depends upon the nature of matter either it is metal or non metal.
Aakash
Basically matter never be a positive or negative. Matter can only be change into postive ion or negative ion.
Aakash
Bromine is a nonmetal liquid. Can you say that Bromine is negatively changed.
Rakesh
Because, Bromine here forming Covalent bond.
Rakesh
Isolated gaseous atom of bromine can be changed in negative charge atom.
Aakash
Practice Key Terms 5

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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