Difference Between Aldehydes and Ketones

Edited by Diffzy | Updated on: September 24, 2022

       

Difference Between Aldehydes and Ketones Difference Between Aldehydes and Ketones

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Introduction

Every school student when offered the prospect of mixing solutions is tempted to conduct their experiments. It is intriguing to mix clear potions and watch a whole different colour swirl in the beaker. These fantasies are often deceptive. We always fail to factor in the strict supervisor that will not let us perform experiments to our hearts’ desire, the nasty smells from the experiments gone wrong (or right) and all the chemical burns that could endanger our lives. It is for these reasons that those few curious minds lose their interest in the challenging field of chemistry. Although, the endless lists of formulae do not particularly plead the chemistry case. 

Chemistry, thus, is not well-liked by many students, which is a pity. The intricacies of the various solutions we come across in our daily lives make the difference between shampoo and conditioner. Even in our food, baking powder and baking soda can make all the difference. They have similar chemical structures but they could be the difference between a fluffy cake and a dense one. Now, you do not want to end up with dense chocolate goo in your mouth, do you? Yes, that is a very likely consequence if you go wrong with those chemical compounds.

Aldehydes vs Ketones

Aldehydes and ketones are two other similar chemical compounds. They both contain a carbonyl group with an oxygen atom attached to a carbon atom with a double bond (C=O). the main difference between them is their chemical structure. While both have the carbonyl group, the arrangement of surrounding molecules changes their constituency. In the aldehyde, the carbonyl group is attached to an alkyl group on one side and a hydrogen atom on the other, while in the ketone compound, the carbonyl group is attached to alkyl groups on either side. There are more differences between the two compounds.

Differences Between Aldehydes and Ketones in a Tabular Form

Table: Aldehydes vs Ketones
Parameters of Comparison
Aldehydes
Ketones
Definition
An aldehyde is an organic compound that has the following general chemical formula – R-CHO.
A ketone is an organic compound that has the following general chemical formula – R-CO-R’.
Etymology
Aldehyde was discovered in 1774 by chemist Carl Wilhelm Scheele. The name was coined by Justus von Liebig from the Modern Latin abbreviation “alcohol dehydrogenatum.”
“Ketone” was coined by German chemist Leopold Gmelin from the German word “aketon”, which means “like acetone”.
Occurrence
Aldehydes mostly occur as volatile compounds and flavouring agents.
Ketones mostly occur in sugars.
Functional group
In aldehydes, the functional group always occurs at the terminal end of the carbon chain of the molecule.
In ketones, the functional group always occurs in the middle of the carbon chain of the molecule.
Location
In aldehydes, the carbonyl group binds with the hydrogen atom at one end.
In ketones, the carbonyl group binds with a carbon atom on both ends.
Nomenclature
Aldehydes bear the suffix “-al”.
Ketones bear the suffix “-one”.
Reactivity
Aldehydes are reactive compounds.
Ketones are relatively less reactive than aldehydes.
Oxidation
Aldehyde compounds undergo oxidation by forming carboxylic acid.
Ketone compounds do not undergo oxidation without breaking the carbon chain.

What is an Aldehyde?

An aldehyde is an organic chemical compound. It has the chemical structure of R-CHO, where C is the carbon atom that forms a double bond with the oxygen atom. The structure, therefore, is as follows – R-C=O-H. The C=O group is the carbonyl group, “R” is the alkyl group (a functional group in organic chemistry that contains only carbon and hydrogen atoms arranged in a chain) and H is the hydrogen atom. Since the hydrogen atom is attached to the carbonyl group, the aldehyde group occurs at the end of the carbon chain.

According to the IUPAC, which stands for the International Union of Pure and Applied Chemistry, the nomenclature of aldehyde compounds is as follows – the compounds bearing the aldehyde group must end with the suffix “-al”. Therefore, the compounds we know as propanal, butanal, hexanal etc. are aldehyde groups with their alkane groups – propane, butane and hexane respectively.

Hydrocarbons (compounds that only contain hydrogen and carbon) generally are non-polar and thus, have low boiling and melting points. The melting and boiling points of carbonyl-containing compounds are relatively higher. For example, butane (a hydrocarbon) and propanal (an aldehyde) have the same molecular weight (58), but the boiling point of butane is 0° C, while the boiling point of propanal is 49° C.

Aldehydes are also known to be reactive. Since they are essential in organic chemistry, they are used to synthesize a lot of other compounds. There are many ways to prepare aldehydes as well. Oxidation is one of the most basic ways to prepare them. Aldehydes are oxidized from alcohols but this is not as easy since the reagents that oxidize aldehydes from alcohol quickly further oxidize them to carboxylic acids. To minimize these reactions catalysts like copper, copper chromite and reagents like pyridinium chlorochromate (large scale) are used.

Aldehydes are extremely useful in our daily lives. Formaldehyde is used massively in the industrial sector. It is used in tanning, embalming and preserving. It is also used in germicides, fungicides and pesticides in plants and animals. However, the largest use of formaldehyde is in the production of polymeric materials. Bakelite, which is a polymer, a plastic, is produced by the reaction between formaldehyde and phenol. It is used not only in plastics but also, and more importantly in adhesives and coatings.

Other than that, aldehydes are used as solvents, flavouring agents and perfumes in the production of various dyes and pharmaceuticals. Some of these aldehydes occur naturally like benzaldehyde, which gives the flavour of almonds, cinnamaldehyde, which gives the flavour of cinnamon and vanillin, which gives the flavour of vanilla beans.

They are even present in the human body. Carbohydrates are compounds that contain aldehydes and ketones with a hydroxyl (-OH) group. Retinal, an aldehyde, combines with opsin (a protein present in the retina) to form rhodopsin. This rhodopsin is responsible for vision.

What is Ketone?

A ketone is also an organic chemical compound like an aldehyde. However, the carbonyl group (C=O) is attached on either side with a carbon atom. Its chemical structure is as follows – R-C=O-R’, where R is the alkyl group that contains the carbon atom. Since the carbonyl group is bound by the alkyl group on both sides, the ketone group is never found at the terminal end of the carbon chain.

The IUPAC name for the compounds containing the ketone group end with the suffix “-one”. For example, butanone, pentanone etc. are ketone compounds with the alkane groups butane and pentane respectively.

Ketones are also highly reactive but not as reactive as aldehydes. They are capable of undergoing a wide variety of chemical reactions. The reason this is possible is due to the carbonyl structure. As already mentioned with the aldehydes, the carbonyl group is highly polar. This is because of the uneven distribution of the electrons. It allows for reduction-oxidation reactions and nucleophilic additions. Since ketones are in their highest oxidation form, they cannot further be oxidized. However, they do easily undergo reduction and can be reduced to the corresponding alcohol.

Ketones possess important physiological properties. They are abundantly found in natural sugars and compounds that are necessary for the production of medical compounds. They are also found in natural and synthetic steroid hormones. The anti-inflammatory compound – cortisone – contains three ketone groups.

Ketones can be manufactured on a large scale with relative ease when compared to aldehydes. They are easy to prepare, have high reactivity and are relatively stable. Many complex compounds are created with ketones as building blocks. These compounds are usually used as solvents in the manufacture of lacquers, paints and even explosives. They are also used as preservatives in tanning and hydraulic fluids.

The most common ketone we come across is acetone. It has a sweetish odour and is soluble in water. It is also known to dissolve many other organic compounds. It has a low boiling point - 56° C – which causes it to be easy to remove since it evaporates when it has served its purpose. This is the main reason why it is one of the most important industrial solvents and has multiple uses in paints, varnishes, nail polish removers, coatings, resins etc.

Main Differences Between Aldehydes and Ketones In Points

Following are the main differences between aldehydes and ketones:

  1. Aldehydes are organic compounds that have the general formula – R-CHO, whereas ketones are organic compounds that have the general formula – R-CO-R’. (In both these formulae “R” is the alkyl group).
  2. Aldehydes were discovered earlier than ketones and were found as the first oxidation product of alcohol. The term “aldehyde” was coined by the chemist Justus von Liebig from the Modern Latin abbreviation “alcohol dehydrogenatum.” The term “ketone” was coined by German chemist Leopold Gmelin from the German word “aketon”, which means “like acetone”.
  3. Aldehydes are much more reactive than ketones.
  4. In aldehydes, the functional group always occurs at the terminal end of the carbon chain of the molecule, while in ketones, the functional group always occurs in the middle of the carbon chain of the molecule.
  5. In aldehydes, the carbonyl group is attached to the hydrogen atom at one end, whereas in ketones, there is a carbon atom attached to either end of the carbonyl group.
  6. Aldehydes have the suffix “-al” in their compounds, while ketones have the suffix “-one” in their compounds.
  7. Aldehyde compounds undergo oxidation by forming carboxylic acids, while ketones do not undergo oxidation unless they break their chain.
  8. Aldehydes are commonly found in volatile compounds and flavouring agents. Ketones are commonly found in sugars.

Conclusion

Aldehydes and ketones are thus very important organic compounds. Aldehyde is a chemical organic compound with the general chemical structure of R-CHO, where it contains a carbonyl group (C=O) and an alkyl group. The carbon atom of the carbonyl group is attached to the alkyl group on one side and a hydrogen atom on the other. Since there is a hydrogen atom attached to it, the aldehyde group is always at the end of the carbon chain o the molecule. The IUPAC name for a compound with an aldehyde group ends with the suffix “-al”. Aldehydes find tremendous use in the industry as formaldehyde, an aldehyde, is key in the manufacture of bakelite, which is used in plastics and other polymers. They are also found in flavouring agents and the human body (retinal).

Ketones similarly have multiple uses too. Although, they are mostly used as a solvent in paints, resins, varnishes etc. They form compounds that find purpose in the pharmaceutical industry and are found in complex sugars and even in steroidal hormones. Ketones are the organic chemical compounds with the general chemical formula – R-C=O-R’- where R is the alkyl group. Here, the carbon atom of the carbonyl group is attached to alkyl groups on either side. Since it is thus attached, the ketone group is always in the middle of the carbon chain of the molecule. For ketones, the IUPAC name of the compound bearing them has the suffix “-one”. Both these compounds share some characteristics but are vastly different in their structures and uses. To know for certain that one is different from the other, there can be tests performed in the laboratory. An aldehyde can be differentiated from a ketone by performing the following tests – the Schiff’s Test (presence of aldehyde produces a magenta colour with the reagent), Tollen’s Test (appearance of metallic silver on the test tube in the presence of aldehyde) and Fehling’s Test (formation of a red precipitate in the presence of an aldehyde). So, you see, aldehydes and ketones are very different and can cause different reactions if they get mixed up. If you come upon these chemical compounds and have no laboratory to test for their structure, do err on the side of caution and steer clear of them. Remember that you are not in school anymore and there isn’t a supervisor watching over you.

References

  1. https://www.etymonline.com/word/aldehyde
  2. https://www.etymonline.com/search?q=ketone

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"Difference Between Aldehydes and Ketones." Diffzy.com, 2022. Sun. 27 Nov. 2022. <https://www.diffzy.com/article/difference-between-aldehydes-and-ketones-365>.



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