Physical Properties of Aldehydes and Ketones

Aldehydes and Ketones show unique physical properties due to the polar (C=O) group. These affect boiling point, solubility, odor, and physical state. Related topics: Nomenclature, Wikipedia, NCERT.

Boiling Points

• Moderate boiling points (higher than alkanes, lower than alcohols).
• Order: alcohols > aldehydes/ketones > alkanes.
• Dipole–dipole interactions exist but no intermolecular H-bonding.
• Ketones usually have slightly higher boiling points than aldehydes.

हिंदी: एल्डिहाइड और कीटोन का क्वथनांक अल्केन से अधिक लेकिन अल्कोहल से कम होता है।

Solubility in Water

• Lower aldehydes/ketones are water-soluble due to H-bonding with water.
• Solubility decreases as alkyl chain length increases.
• Examples: formaldehyde, acetaldehyde, acetone (miscible in water).

हिंदी: छोटे एल्डिहाइड और कीटोन पानी में घुलनशील होते हैं, परंतु बड़े चेन वाले कम घुलनशील होते हैं।

Odor

• Lower aldehydes/ketones → sharp, pungent, or sweet odor.
• Formaldehyde = strong, irritating smell; Acetone = sweet fruity odor.
• Some ketones (like muscone) → pleasant fragrance.

हिंदी: छोटे एल्डिहाइड और कीटोन की गंध तीखी या मीठी होती है।

Physical State

• Formaldehyde → gas, Acetaldehyde & Acetone → volatile liquids.
• Higher aldehydes/ketones → waxy solids or oily liquids.
• Mostly colorless in pure form.

हिंदी: छोटे सदस्य वाष्पशील द्रव होते हैं, जबकि बड़े सदस्य ठोस या तैलीय होते हैं।

Polarity and Intermolecular Forces

• Carbonyl group is highly polar due to electronegative oxygen.
• Exhibit strong dipole-dipole interactions.
• No intermolecular H-bonding among themselves.

हिंदी: कार्बोनिल समूह ध्रुवीय होता है और डाइपोल-डाइपोल आकर्षण दिखाता है।

MCQs

1. Boiling point order is:
   a) Alcohols > Aldehydes > Alkanes
   b) Aldehydes > Alcohols > Alkanes
   c) Alkanes > Aldehydes > Alcohols
   ✅ Answer: (a)
2. Which ketone has a fruity odor?
   a) Formaldehyde
   b) Acetone
   c) Butanal
   d) Benzaldehyde
   ✅ Answer: (b)

True/False

• Ketones form intermolecular hydrogen bonds with themselves. ❌ False
• Formaldehyde is a gas at room temperature. ✅ True

Fill in the Blanks

• Lower aldehydes and ketones are _______ in water. ✅ Miscible
• The _______ group is responsible for polarity. ✅ Carbonyl

Quick Quiz

Q: Why do ketones generally have higher boiling points than aldehydes of similar mass?
Ans: Ketones are more symmetrical and show stronger dipole-dipole interactions.