ChemistryChapter 18: Alcohol
Iodoform Test (named)
Iodoform: yellow ppt CHI3, antiseptic smell named.
Learning objectives
- Explain iodoform test (named) in simple chemical language.
- Connect 1. positive with 2. negative using equations, observations, or trends where needed.
- Answer a short Federal Board question with the correct terms and a clear reason.
Definition
Iodoform: yellow ppt CHI3, antiseptic smell named. Positive for ethanal, methyl ketones (CH3COR), and alcohols that oxidise to those (ethanol, propan-2-ol). Needs a CH3CH(OH)- or CH3CO- fragment. Methanol negative. Meaning: a structure clue, not a medical dose (old antiseptic slogan only). I2 / alkali named reagents, not a pharmacy recipe. Methyl carbonyl or ethanol gives yellow CHI3 precipitate with I2/OH-. Primary alcohols need distillation to stop at aldehyde; dichromate colour change should be stated.
Important terms
- IODOFORM TEST (NAMED)
- Iodoform: yellow ppt CHI3, antiseptic smell named.
- 1. POSITIVE
- Ethanol (oxidises to ethanal).
- 2. NEGATIVE
- Methanol.
Overview
Who gives it, then who does not:
Positive
Ethanol (oxidises to ethanal). Propan-2-ol (to propanone). Propanone. Ethanal. The CH3 next to C=O / CH(OH) is the tell. Acetaldehyde and ethanol positive; methanol negative — discrimination.
Examples Yellow ppt · Smell named · Ethanol yes
Negative
Methanol. Propan-1-ol (gives propanal, no methyl ketone). Too-long acids. Use it with other tests, not alone. Named test for CH3CH(OH)- or CH3CO- group.
Examples Methanol no · Propan-1-ol no · Need the fragment
WHAT — IODOFORM TEST (NAMED)
Iodoform: yellow ppt CHI3, antiseptic smell named. Positive for ethanal, methyl ketones (CH3COR), and alcohols that oxidise to those (ethanol, propan-2-ol). Needs a CH3CH(OH)- or CH3CO- fragment. Methanol negative. Meaning: a structure clue, not a medical dose (old antiseptic slogan only). I2 / alkali named reagents, not a pharmacy recipe. Methyl carbonyl or ethanol gives yellow CHI3 precipitate with I2/OH-. Primary alcohols need distillation to stop at aldehyde; dichromate colour change should be stated.
WHY — Why this topic matters
IODOFORM TEST (NAMED) is an independent FBISE learning unit because students must use its definitions, conditions, equations, or trends separately in explanations and calculations.
HOW — How it works
POSITIVE: Ethanol (oxidises to ethanal). Propan-2-ol (to propanone). Propanone. Ethanal. The CH3 next to C=O / CH(OH) is the tell. Acetaldehyde and ethanol positive; methanol negative — discrimination. Examples: Yellow ppt; Smell named; Ethanol yes. NEGATIVE: Methanol. Propan-1-ol (gives propanal, no methyl ketone). Too-long acids. Use it with other tests, not alone. Named test for CH3CH(OH)- or CH3CO- group. Examples: Methanol no; Propan-1-ol no; Need the fragment. Exam iodoform: ethanol yes (yellow ppt CHI3) methanol no needs the CH3CH(OH)- or CH3CO- piece
In simple words
Named test. No antiseptic dosing, no iodine recipes to misuse.
Key points
- yellow CHI3 ppt
- needs methyl-carbonyl or ethanol-type
- CH3-C=O story
- no CH3CO made
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Iodoform Test (named)
Sticky Notes · Quick Revision
Core Idea
- Iodoform: yellow ppt CHI3, antiseptic smell named. Positive for ethanal, methyl ketones (CH3COR), and alcohols that oxidise to those (ethanol, propan-2-ol). Needs a CH3CH(OH)- or CH3CO- fragment. Methanol negative. Meaning: a structure clue, not a medical dose (old antiseptic slogan only). I2 / alkali named reagents, not a pharmacy recipe. Methyl carbonyl or ethanol gives yellow CHI3 precipitate with I2/OH-. Primary alcohols need distillation to stop at aldehyde; dichromate colour change should be stated.
1. POSITIVE
- Ethanol (oxidises to ethanal). Propan-2-ol (to propanone). Propanone. Ethanal. The CH3 next to C=O / CH(OH) is the tell. Acetaldehyde and ethanol positive; methanol negative — discrimination.
- Yellow ppt
- Smell named
- Ethanol yes
2. NEGATIVE
- Methanol. Propan-1-ol (gives propanal, no methyl ketone). Too-long acids. Use it with other tests, not alone. Named test for CH3CH(OH)- or CH3CO- group.
- Methanol no
- Propan-1-ol no
- Need the fragment
Exam Example
- Exam iodoform:
- ethanol yes (yellow ppt CHI3)
- methanol no
- needs the CH3CH(OH)- or CH3CO- piece
Common Misconceptions
- Do not confuse 1. POSITIVE with 2. NEGATIVE; state the difference before giving an example.
- Writing only “yellow CHI3 ppt” is not enough. Explain what it means in a complete sentence.
FBISE Exam Check
- Explain iodoform test (named), compare its two main parts, and include the correct chemical evidence.
- Named test. No antiseptic dosing, no iodine recipes to misuse.
60-second revision
- Iodoform: yellow ppt CHI3, antiseptic smell named.
- 1. POSITIVE: Ethanol (oxidises to ethanal).
- 2. NEGATIVE: Methanol.
- Exam reminder: Exam iodoform: ethanol yes (yellow ppt CHI3) methanol no needs the CH3CH(OH)- or CH3CO- piece
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