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Chemistry · Aldehydes, Ketones and Carboxylic Acids

Nomenclature & Structure of the Carbonyl Group

The carbonyl group, $\ce{>C=O}$, is the single most important functional group in the organic chemistry of Class 12, and NCERT Section 8.1 opens the whole unit with it. Master two things here and the rest of aldehyde-and-ketone chemistry falls into place: the electronic structure of the $\ce{C=O}$ bond, which dictates every reaction, and the naming rules that let you decode any carbonyl compound a NEET stem can throw at you.

What the Carbonyl Group Is

In the previous unit you studied organic compounds with a carbon–oxygen single bond. The carbonyl group is the carbon–oxygen double bond, written $\ce{>C=O}$, and it is the defining feature of three families NEET examines heavily: aldehydes, ketones and carboxylic acids. NCERT calls it “one of the most important functional groups in organic chemistry,” and the compounds that carry it — from acetone the solvent to vanillin the flavour — are constituents of fabrics, drugs, plastics and perfumes.

The identity of a carbonyl compound is set entirely by what the carbonyl carbon is bonded to. When it is joined to at least one hydrogen the compound is an aldehyde; when it is joined to two carbons (alkyl or aryl) it is a ketone; and when it also carries an $\ce{-OH}$ it is a carboxylic acid.

FamilyCarbonyl carbon bonded toGeneral formulaExample
AldehydeOne C (or H) and one H$\ce{R-CHO}$$\ce{CH3CHO}$ (ethanal)
KetoneTwo C atoms (alkyl/aryl)$\ce{R-CO-R'}$$\ce{CH3COCH3}$ (propanone)
Carboxylic acidOne C (or H) and $\ce{-OH}$$\ce{R-COOH}$$\ce{CH3COOH}$ (ethanoic acid)

The simplest aldehyde, formaldehyde $\ce{HCHO}$, is unusual in that the carbonyl carbon carries two hydrogens; every other aldehyde has the pattern $\ce{R-CHO}$. A ketone whose two groups are identical (such as $\ce{CH3COCH3}$) is a symmetrical ketone; if they differ it is unsymmetrical. This article concentrates on aldehydes and ketones, the “simplest and most important carbonyl compounds”; acid naming is treated in the sibling note on carboxylic acids.

Structure and Bonding of >C=O

The carbonyl carbon is $sp^2$ hybridised. Three of its valence orbitals are $sp^2$ hybrids that form three sigma ($\sigma$) bonds — one to oxygen and two to the attached groups. The fourth valence electron stays in an unhybridised $p$ orbital, which overlaps sideways with a $p$ orbital of oxygen to form the pi ($\pi$) bond. The $\ce{C=O}$ double bond is therefore one $\sigma$ plus one $\pi$ bond, exactly analogous to $\ce{C=C}$ in alkenes.

Because the carbon uses three $sp^2$ orbitals in a plane, the carbonyl carbon and the three atoms attached to it are coplanar, and the bond angles are approximately $120^\circ$, as expected of a trigonal-planar (trigonal coplanar) arrangement. The $\pi$-electron cloud sits above and below this plane. Oxygen, in addition to its bonds, carries two non-bonding lone pairs.

Figure 1

Orbital picture and polarity of the carbonyl group

C O R H ~120° δ+ δ− σ bond π bond lone pairs p orbital

The carbon and its three attached atoms lie in one plane at about $120^\circ$; the unhybridised $p$ orbitals (purple) overlap above and below the plane to give the $\pi$ bond. Oxygen pulls electron density toward itself, leaving carbon $\delta+$ and oxygen $\delta-$.

Polarity, Dipole and Resonance

Oxygen is considerably more electronegative than carbon, so the $\ce{C=O}$ bond is strongly polarised: bonding electrons are drawn toward oxygen. This makes the carbonyl carbon an electrophilic, Lewis-acidic centre ($\delta+$) and the carbonyl oxygen a nucleophilic, Lewis-basic centre ($\delta-$). That single fact is the engine of nucleophilic-addition chemistry: nucleophiles attack the electron-poor carbon, electrophiles approach the electron-rich oxygen.

Carbonyl compounds carry substantial dipole moments and are more polar than ethers of comparable size. NCERT explains the high polarity through resonance between a neutral structure (A) and a charge-separated dipolar structure (B):

$\ce{R2C=O <-> R2\overset{+}{C}-\overset{-}{O}}$

Structure B places a full positive charge on carbon and a full negative charge on oxygen. The true molecule is a resonance hybrid weighted toward A, but the contribution of B raises the dipole moment and confirms the electron-poor character of the carbonyl carbon. This electronic profile also explains physical trends — dipole–dipole attraction raises the boiling point of carbonyls above comparable hydrocarbons, though the absence of $\ce{O-H}$ keeps it below comparable alcohols.

NEET Trap

Geometry vs. hybridisation confusion

Students sometimes label the carbonyl carbon $sp$ or $sp^3$. It is $sp^2$: three $\sigma$ bonds plus one $\pi$ bond, trigonal planar at $\sim120^\circ$. An $sp$ centre (as in $\ce{HCN}$ or alkynes) would be linear with two $\pi$ bonds; an $sp^3$ centre has no leftover $p$ orbital and cannot form the $\pi$ bond a $\ce{C=O}$ requires.

Three $\sigma$ + one $\pi$ ⇒ $sp^2$ ⇒ planar, $\sim120^\circ$. Carbon is always the $\delta+$ site.

Common Names of Aldehydes

Aldehydes and ketones are often called by common names rather than IUPAC names, and NEET reaction schemes use both freely, so you must recognise each. The common name of an aldehyde is derived from the common name of the corresponding carboxylic acid by replacing the ending -ic of the acid with -aldehyde. These acid names trace back to a Latin or Greek word for the natural source of the acid (for example formica, ant, gives formic acid and hence formaldehyde).

The position of a substituent in the common system is shown by the Greek letters $\alpha,\ \beta,\ \gamma,\ \delta$ — the $\alpha$-carbon being the one directly attached to the $\ce{-CHO}$ group, the $\beta$-carbon the next, and so on. Thus $\ce{CH3CH(OCH3)CHO}$ is $\alpha$-methoxypropionaldehyde (the $\ce{-OCH3}$ sits on the carbon next to $\ce{-CHO}$).

StructureCommon nameSource word
$\ce{HCHO}$Formaldehydeformic (ant)
$\ce{CH3CHO}$Acetaldehydeacetic (vinegar)
$\ce{CH3CH2CHO}$Propionaldehydepropionic
$\ce{CH3CH2CH2CHO}$Butyraldehydebutyric (butter)
$\ce{(CH3)2CHCHO}$Isobutyraldehydebutyric

IUPAC Names of Aldehydes

The IUPAC name of an open-chain aliphatic aldehyde is built from the corresponding alkane by replacing the final -e with -al. The longest carbon chain is numbered starting from the carbon of the $\ce{-CHO}$ group, so the carbonyl carbon is always C1. Because its locant is fixed at 1, it is never written in the name. Substituents are cited as prefixes in alphabetical order, each with the numeral of its position.

When the $\ce{-CHO}$ group is attached to a ring, the carbonyl carbon is not part of the ring chain, so the -al ending cannot be used. Instead the suffix -carbaldehyde is added after the full name of the cycloalkane, and the ring carbon bearing the group is numbered 1 (for example, 3-methylcyclohexanecarbaldehyde). Likewise the simplest aromatic aldehyde is benzenecarbaldehyde, though its common name benzaldehyde is also accepted by IUPAC.

Worked Example

Name $\ce{CH3CH(CH3)CHO}$ and explain why the locant of the carbonyl is not stated.

The longest chain through the $\ce{-CHO}$ has three carbons (propanal skeleton). Numbering from the aldehyde carbon: C1 = CHO, C2 carries a methyl, C3 = methyl end. So the name is 2-methylpropanal. The carbonyl carbon is fixed at C1 by rule, so “1” is omitted — we never write “propanal-1”. (Common name: isobutyraldehyde.)

Build on this

The $\delta+$ carbonyl carbon is exactly where nucleophiles strike. See how that plays out in Nucleophilic Addition to the Carbonyl Group.

Common Names of Ketones

The common name of a ketone is formed by naming the two alkyl or aryl groups attached to the carbonyl group, in alphabetical order, followed by the word ketone. So $\ce{CH3COCH2CH2CH3}$ is methyl n-propyl ketone and $\ce{(CH3)2CHCOCH(CH3)2}$ is diisopropyl ketone. Substituent positions in the common system use Greek letters $\alpha,\ \alpha',\ \beta,\ \beta'$ starting from the carbons on either side of the carbonyl.

A few ketones keep historical common names: the simplest, dimethyl ketone $\ce{CH3COCH3}$, is universally called acetone. These names are worth memorising because they recur in solvent and reaction contexts.

IUPAC Names of Ketones

For ketones the IUPAC name comes from the parent alkane by replacing the final -e with -one. Unlike an aldehyde, a ketone carbonyl is in the interior of the chain, so its position must be specified. The chain is numbered from the end nearer the carbonyl group so that the carbonyl carbon receives the lowest possible locant, which is then quoted before the -one suffix.

Cyclic ketones follow the same logic: the carbonyl carbon of the ring is numbered 1, so cyclohexanone needs no locant, while a substituted ring such as 2-methylcyclohexanone does. NCERT lists examples including pentan-2-one (methyl n-propyl ketone) and 2,4-dimethylpentan-3-one (diisopropyl ketone).

NEET Trap

Lowest-locant rule for the carbonyl

In $\ce{CH3COCH2CH2CH3}$, numbering from the right would put the carbonyl at C4, but numbering from the left (nearer carbon) puts it at C2. The carbonyl always wins the lowest locant, so the name is pentan-2-one, never “pentan-4-one.” For aldehydes this never arises because $\ce{-CHO}$ is forced to C1.

Aldehyde → number from $\ce{-CHO}$ (C1, omit). Ketone → number to give $\ce{C=O}$ the lowest locant (state it).

Naming Aromatic Carbonyls

Aromatic aldehydes are named as substituted benzaldehydes, with the ring carbon bearing $\ce{-CHO}$ numbered 1. So m-bromobenzaldehyde is 3-bromobenzaldehyde (equivalently 3-bromobenzenecarbaldehyde), and benzene-1,2-dicarbaldehyde is the systematic name for phthalaldehyde.

For alkyl aryl ketones, the common system attaches the acyl-group name as a prefix to the word -phenone. The most important is acetophenone, $\ce{C6H5COCH3}$ (methyl phenyl ketone; IUPAC: 1-phenylethan-1-one). When the carbonyl bridges two phenyl rings the compound is benzophenone, $\ce{C6H5COC6H5}$ (diphenyl ketone).

CompoundStructureCommon nameIUPAC name
Simplest aromatic aldehyde$\ce{C6H5CHO}$BenzaldehydeBenzenecarbaldehyde
Methyl phenyl ketone$\ce{C6H5COCH3}$Acetophenone1-Phenylethan-1-one
Diphenyl ketone$\ce{C6H5COC6H5}$BenzophenoneDiphenylmethanone
Figure 2

Reading three named carbonyl structures

Pentan-2-one O 1 2 3 4 5 Benzaldehyde O H Acetophenone O CH₃

Left: a ketone numbers to give $\ce{C=O}$ the lowest locant (C2). Centre and right: aromatic carbonyls keep the $\ce{-CHO}$ or acyl group on the ring — benzaldehyde ($\ce{C6H5CHO}$) and acetophenone ($\ce{C6H5COCH3}$).

Name Reference Table

The compounds below recur across NEET reaction schemes and matching questions. Knowing the common-to-IUPAC mapping cold removes a major source of careless error.

StructureCommon nameIUPAC name
$\ce{HCHO}$FormaldehydeMethanal
$\ce{CH3CHO}$AcetaldehydeEthanal
$\ce{CH3CH2CHO}$PropionaldehydePropanal
$\ce{(CH3)2CHCHO}$Isobutyraldehyde2-Methylpropanal
$\ce{CH3CH2CH2CH2CHO}$ValeraldehydePentanal
$\ce{CH3COCH3}$Acetone (dimethyl ketone)Propanone
$\ce{CH3COCH2CH2CH3}$Methyl n-propyl ketonePentan-2-one
$\ce{(CH3)2CHCOCH(CH3)2}$Diisopropyl ketone2,4-Dimethylpentan-3-one
$\ce{C6H5CHO}$BenzaldehydeBenzenecarbaldehyde
$\ce{C6H5COCH3}$Acetophenone1-Phenylethan-1-one
Quick Recap

Carbonyl nomenclature & structure in one screen

  • Carbonyl carbon is $sp^2$: three $\sigma$ bonds, one $\pi$ bond, planar at $\sim120^\circ$; oxygen has two lone pairs.
  • $\ce{C=O}$ is polar — carbon $\delta+$ (electrophilic, Lewis acid), oxygen $\delta-$ (nucleophilic, Lewis base); resonance $\ce{R2C=O <-> R2\overset{+}{C}-\overset{-}{O}}$ raises the dipole.
  • Aldehyde IUPAC: alkane $-e \to -al$; $\ce{-CHO}$ carbon is C1 (locant omitted). Ring $\ce{-CHO} \to$ -carbaldehyde.
  • Ketone IUPAC: alkane $-e \to -one$; number for the lowest carbonyl locant and quote it (pentan-2-one).
  • Aromatics: benzaldehyde ($\ce{C6H5CHO}$), acetophenone ($\ce{C6H5COCH3}$, methyl phenyl ketone), benzophenone ($\ce{C6H5COC6H5}$, diphenyl ketone).

NEET PYQ Snapshot — Nomenclature & Structure of the Carbonyl Group

NEET rarely asks naming in isolation, but the electronic structure of $\ce{C=O}$ underpins recurring concept questions. Below is a real PYQ on carbonyl behaviour plus two concept checks built on the structural ideas above.

NEET 2016 · Q.13

The correct statement regarding a carbonyl compound with a hydrogen atom on its alpha-carbon is:

  1. it equilibrates with its enol, a process called aldehyde-ketone equilibration
  2. it equilibrates with its enol, a process called carbonylation
  3. it rapidly equilibrates with its corresponding enol, known as keto-enol tautomerism
  4. it never equilibrates with its corresponding enol
Answer: (3)

A carbonyl compound with an $\alpha$-hydrogen interconverts with its enol form — this is keto-enol tautomerism. The $\alpha$-carbon is, by definition, the carbon directly attached to the carbonyl group, exactly the labelling rule introduced for common names. Tautomerism is possible only because the $\ce{C=O}$ is polarised and the $\alpha$-$\ce{C-H}$ is weakly acidic.

Concept

The carbonyl carbon and the carbonyl oxygen are best described, respectively, as:

  1. nucleophilic ($\delta-$) and electrophilic ($\delta+$)
  2. electrophilic ($\delta+$) and nucleophilic ($\delta-$)
  3. both electrophilic
  4. non-polar centres
Answer: (2)

Oxygen's higher electronegativity polarises the bond, leaving carbon $\delta+$ (a Lewis-acidic, electrophilic site that nucleophiles attack) and oxygen $\delta-$ (a Lewis-basic, nucleophilic site). This is the single most exam-relevant consequence of carbonyl structure.

Concept

The IUPAC name of $\ce{CH3COCH2CH2CH3}$ is:

  1. pentan-4-one
  2. methyl propyl ketone
  3. pentan-2-one
  4. pentanal
Answer: (3)

The five-carbon chain is numbered to give the carbonyl the lowest locant, so $\ce{C=O}$ is at C2, giving pentan-2-one. “Methyl propyl ketone” is the common (not IUPAC) name, and “pentanal” would be the aldehyde, not the ketone.

FAQs — Nomenclature & Structure of the Carbonyl Group

Common doubts on carbonyl bonding and naming, answered for NEET.

Why is the carbonyl carbon sp2 hybridised and not sp3?
The carbonyl carbon forms three sigma bonds and one pi bond. Three sigma bonds require three equivalent hybrid orbitals, which come from mixing one s and two p orbitals to give three sp2 orbitals in a plane at about 120 degrees. The remaining unhybridised p orbital is left perpendicular to that plane and overlaps sideways with a p orbital of oxygen to form the pi bond of the C=O double bond. An sp3 carbon has no leftover p orbital for a pi bond, so it cannot form the double bond.
Why is the carbonyl group polar, and which atom carries the partial positive charge?
Oxygen is more electronegative than carbon, so the shared electrons of the C=O bond are pulled toward oxygen. The carbonyl carbon therefore carries a partial positive charge (delta plus) and behaves as an electrophilic, Lewis-acidic centre, while the oxygen carries a partial negative charge (delta minus) and is a nucleophilic, Lewis-basic centre. The high polarity is reinforced by resonance between a neutral structure and a dipolar structure that places a full positive charge on carbon and a full negative charge on oxygen.
How do you derive the IUPAC name of an aliphatic aldehyde and a ketone?
Take the name of the alkane with the longest chain containing the carbonyl carbon. For an aldehyde, replace the final -e with -al; the carbon of the CHO group is always C1, so its locant is not written. For a ketone, replace the final -e with -one and number the chain from the end that gives the carbonyl carbon the lowest locant, then quote that locant before -one. For example CH3CH2CHO is propanal and CH3COCH2CH2CH3 is pentan-2-one.
When is the suffix carbaldehyde used instead of -al?
When the CHO group is attached directly to a ring, the carbon of CHO is not counted as part of the ring chain, so the -al ending cannot be used. Instead the suffix -carbaldehyde is added after the full name of the cycloalkane or arene, and the ring carbon bearing the CHO group is numbered 1. For example, cyclohexanecarbaldehyde and benzenecarbaldehyde (the latter is commonly called benzaldehyde, a name IUPAC also accepts).
What is the difference between acetophenone and benzophenone?
Both are aromatic ketones. Acetophenone is methyl phenyl ketone, in which a carbonyl group joins one methyl group and one phenyl group (C6H5COCH3). Benzophenone is diphenyl ketone, in which the carbonyl group joins two phenyl groups (C6H5COC6H5). Alkyl aryl ketones such as acetophenone are commonly named by attaching the acyl-group name as a prefix to the word -phenone.
Are common names like formaldehyde and acetone still important for NEET?
Yes. NEET questions, reaction schemes and textbooks freely mix common and IUPAC names, so you must recognise both. Formaldehyde is methanal, acetaldehyde is ethanal, acetone (dimethyl ketone) is propanone and acetophenone is 1-phenylethan-1-one. Knowing the common-to-IUPAC mapping prevents matching-type and reaction-identification errors.
Related Topics
Aldehydes, Ketones and Carboxylic Acids Back to the full chapter hub and all subtopics. Preparation of Aldehydes & Ketones Oxidation, ozonolysis, Rosenmund, Etard and more. Nucleophilic Addition How the δ+ carbonyl carbon reacts with nucleophiles. Physical Properties of Carbonyls Boiling points, polarity and solubility trends. Carboxylic Acid Naming & Prep The −COOH group: nomenclature and preparation. Organic Basic Principles Hybridisation, resonance and IUPAC fundamentals.