Alkenes and Ketones

Topics: Oxygen, Alcohol, Carbonyl Pages: 7 (1169 words) Published: June 28, 2013
Notes Aldehydes and Ketones

The major similarity between an aldehyde and a ketone is the carbonyl group. A carbonyl group is a carbon atom doubly bonded to an oxygen atom.

[pic]
Both molecules have a carbonyl group, the difference the number of carbons bonded to the carbonyl carbon. An aldehyde will have none or one and a ketone will have two carbons.

All aldehydes, except formaldehyde, will have a hydrogen atom on one side of the carbonyl carbon and at least on carbon on the other side. [pic]
All ketones have a carbon on each side of the carbonyl carbon. [pic]

Remember that the ‘R’ symbolizes any carbon side-chain, from one to a million carbons. Basically, what it comes down to is that in an aldehyde the carbonyl group is on the terminal (last) carbon and the ketones carbonyl group is not.

These compounds are found at the most fundamental levels of biological existence. Glucose is the single most important molecule in providing energy at a cellular level. Without glucose you would die in seconds. Glucose, the most important carbohydrate, not only has a carbonyl group but is an aldehyde. Another common carbohydrate is fructose, fruit sugar, this compound is a ketone.

These compounds are more reactive than your typical alkane, the question you may ask is why? The answer lies in the location of the electrons in the carbonyl group. First, look at the hybridization of a carbonyl carbon. A carbon connected to three other molecules must be doubly bonded to one of those molecules. For a double bond to form p-orbitals must overlap over a sigma bond. The hybridization loses one p-orbital, leaving the carbon as sp2, allowing the formation of the other bond with the free p-orbital, forming a pi-bond.

Back to our question, why are the aldehyde and a ketone more reactive than an alkane. When the pi-bond forms the electrons in this molecular orbital are more exposed, making them more vulnerable to reacting. Try to visualize the electrons sticking out on each side of the bond, leaving them accessible to other compounds.

Nomenclature:

Aldehydes – IUPAC Names
1. Count the number of carbons in the longest chain containing the aldehyde group 2. The carbonyl carbon will always be carbon number one
3. Drop the –e suffix and add –al

Examples:
|[pic] |[pic] |[pic] |[pic] | |ethanal |propanal |butanal |octanal |

Aldehydes – Common Names
1. Count the number of carbons
2. Use the side-chain abbreviation
3. Add the word aldehyde to the end

Examples:
|[pic] |[pic] |[pic] |[pic] | |ethyl aldehyde |propyl aldehyde |butyl aldehyde |octyl aldehyde |

Aldehydes – Very Common Names
Just as H2O is referred to as water some aldehydes have very common names. The following is a list of these very specific common names.

|[pic] |[pic] |[pic] | |formaldehyde |acetaldehyde |benzaldehyde |

Ketones – IUPAC Names
1. Count the number of carbons in the longest chain containing the ketone group 2. The carbonyl carbon will always be given the lowest possible number 3. Drop the –e suffix and add –one

Examples:
|[pic] |[pic] |[pic] |[pic] | |propanone |butanone |3-pentanone...
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