Aldehyde and ketone in the same molecule: which gets the suffix, and what does 'oxo' mean?

I love how aldehydes and ketones have those neat names, until a molecule has two functional groups and I forget which one is the boss.

Example: $OHC-CH_2-CO-CH_3$ . Is it 3-oxobutanal or 1-oxo... or something with "al-one"? And what about $CH_3COCH_2COOH$ where there's a ketone and an acid? I have seen the word "oxo" in a name but I can't tell if it's the ketone or the aldehyde that becomes "oxo", and what is "formyl" then?

I also want to make sure I understand the carbonyl group itself. NCERT says the carbon is sp² hybridised and the group is planar and polar. Why does the polarity make carbonyl compounds react with nucleophiles, and why are aldehydes more reactive than ketones?

Lastly, common names. I know formaldehyde and acetone, but I keep mixing acetophenone and benzophenone. Anything that gives me a hook?

My plan is to test myself on 10 names a day, so examples I can do and check are welcome!

7 answers & replies

[+]Nisha Verma12thSample· 2mo ago

Seniority order: carboxylic acid > aldehyde > ketone > alcohol. The senior group gets the suffix; the lower ones become prefixes: ketone/aldehyde carbonyl as a substituent is "oxo", –OH is "hydroxy".

$OHC-CH_2-CO-CH_3$ : aldehyde is senior, so butanal; ketone becomes oxo at C-3. Name: 3-oxobutanal.
$CH_3COCH_2COOH$ : acid is senior, so butanoic acid with oxo at 3: 3-oxobutanoic acid.
$OHC-CH_2CH_2-CO-CH_3$ : 4-oxopentanal.

The aldehyde carbon is always C-1, so no locant is written for it.

[+][–]Pooja Iyer12thSample· 2mo agoAuthor
3-oxobutanal, 3-oxobutanoic acid, 4-oxopentanal, logging those three as my first test set. thank you!

[+]Yash Tiwari11thSample· 2mo ago

bhai ketone ka priority zyada hota hai na, kyunki ketone mein carbon dono side se bonded hai? to 3-oxobutanal ki jagah 3-formylbutan-2-one jaisa kuch aana chahiye? I think so

[+]Arjun Nair12thSample· 2mo ago

Yash, no. Aldehyde is senior to ketone. "Number of carbons attached" isn't the criterion; the order is a fixed list in IUPAC. Also, "formyl" is the name for –CHO used as a substituent when the aldehyde cannot be the suffix, for example when a higher group like a carboxylic acid is present: 3-formylbenzoic acid. In your example the aldehyde is the senior group, so it takes the suffix and the ketone becomes oxo. If an aldehyde group is attached to a ring, it is named with "carbaldehyde", for instance cyclohexanecarbaldehyde.

[+][–]Yash Tiwari11thSample· 2mo ago
oho, fixed list hai, number of carbon ka kuch lena dena nahi. ab yaad rahega. thanks arjun bhaiya

[+]Tanvi Deshmukh12thSample· 2mo ago

Carbonyl group notes!! The carbonyl carbon is sp² hybridised, with three sigma bonds in one plane at about 120° and one pi bond to oxygen. Oxygen is more electronegative so the C=O is polarised: carbon is $\delta+$ , oxygen $\delta-$ . That electrophilic carbon is attacked by nucleophiles, which is why nucleophilic addition is the characteristic reaction.

Aldehydes > ketones in reactivity because (1) the ketone has two alkyl groups which push electrons and reduce the positive charge on carbon (+I effect), and (2) they crowd the carbon, so the nucleophile has a harder time approaching (steric hindrance). Aldehydes have only one alkyl group and one small H.

[+]Kavya NaikDropperSample· 2mo ago

Common names:

Structure	Common name	IUPAC
HCHO	formaldehyde	methanal
CH3CHO	acetaldehyde	ethanal
CH3COCH3	acetone	propanone
C6H5CHO	benzaldehyde	benzaldehyde
C6H5COCH3	acetophenone	1-phenylethanone
C6H5COC6H5	benzophenone	diphenylmethanone

Hook: aceto-phenone is the ACETyl group (CH3CO) on a phenyl; benzo-phenone has two phenyls with the carbonyl in the middle.

Class 12: how do I split the last 100 days between boards and NEET?

Okay so it's officially under 100 days to my first board paper Chennai, PCB, CBSE and I am having a full dramatic crisis, please send help 😭. My situation: prelims are in about three weeks, practicals right after, then b

🎯 Exams & Study❓ Question2 min read👏 7💬 12

Pooja Iyer12thSample· 16d ago

Glucose has an aldehyde group but doesn't give Schiff's test or NaHSO3 addition. Plus, what do alpha and beta actually differ in??

Okay drama alert 🎭 glucose is betraying me. NCERT shows glucose reacting with hydroxylamine to give an oxime and with HCN to give a cyanohydrin, and with bromine water to give gluconic acid, so it clearly has an aldehyde

🧪 NEET Chemistry❓ Question1 min read👏 7💬 7

Pooja Iyer12thSample· 22d ago

What exactly counts as "reproductive health" and how is the RCH programme different from family planning?

Okay, drama aside, this chapter is one of those "easy" ones where the questions are about definitions and I always lose marks to wording. 😭 NCERT's first lines: reproductive health means a total well-being in all aspects

🧬 NEET Biology❓ Question1 min read👏 7💬 7

Keep reading

Kavya NaikDropperSample· 3d ago

Arrhenius: my Ea for 'rate doubles per 10 K' is 53.6 kJ but the book says 52.9. Who is right?

Also what does the factor P actually do and what does a catalyst change?

🧪 NEET Chemistry❓ Question1 min read👏 6💬 6

Nisha Verma12thSample· 4d ago

Benzaldehyde gives Tollens' test but not Fehling's, and which compounds give the iodoform test?

Revision month, chemistry, distinction tests. Every board and NEET paper has at least one "which reagent distinguishes X from Y" question, so I need this airtight. The thing I got wrong in a mock: "Benzaldehyde and propa

🧪 NEET Chemistry❓ Question1 min read👏 7💬 7

Kavya NaikDropperSample· 5d ago

Green chemistry examples in NCERT: which process replaced which (dry cleaning, bleaching, ethanal)? Mixing up the chemicals

Amma says I study chemistry like I'm doing a quiz show, fast but with mistakes 😅. Soil pollution and green chemistry is one of the "easy" last pages that I still get wrong in tests because the examples blur. What I know:

🧪 NEET Chemistry❓ Question1 min read👏 7💬 6