Stereochemistry: Optical purity| NMR spectrum signal count

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Q.4. An optically pure organic compound has a specific rotation of +40^o. The optical purity of the sample that exhibits a specific rotation of +32^o is

1. 8%
2. 12%
3. 20%
4. 80%

Answer: Option 4: 80%

Question number-1 Stereochemistry: Optical purity

Hint:

Given: 𝑆𝑝𝑎𝑐𝑖𝑓𝑖𝑐 𝑟𝑜𝑡𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑝𝑢𝑟𝑒 𝑜𝑟𝑔𝑎𝑛𝑖𝑐 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑 = +40^o
and 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑟𝑜𝑡𝑎𝑡𝑖𝑜𝑛 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑎𝑚𝑝𝑙𝑒 = +32^o

We know that,
The optical purity of the sample = (𝑺𝒑𝒆𝒄𝒊𝒇𝒊𝒄 𝒓𝒐𝒕𝒂𝒕𝒊𝒐𝒏 𝒐𝒇 𝒕𝒉𝒆 𝒔𝒂𝒎𝒑𝒍𝒆 )/(𝑺𝒑𝒂𝒄𝒊𝒇𝒊𝒄 𝒓𝒐𝒕𝒂𝒕𝒊𝒐𝒏 𝒐𝒇 𝒑𝒖𝒓𝒆 𝒐𝒓𝒈𝒂𝒏𝒊𝒄 𝒄𝒐𝒎𝒑𝒐𝒖𝒏𝒅) ×𝟏𝟎𝟎

The optical purity of the sample = 32/40×100 = 80%

The answer to Question number-1 Stereochemistry: Optical purity

Q.5. The number of signals observed in the proton decoupled ¹³C NMR spectrum of the following compound is

1. five
2. six
3. ten
4. thirteen

Question number-2 Stereochemistry: 13C NMR spectrum signal count

Answer: Option 1: Five

Hint:

The Number of signals observed in the proton decoupled 13C NMR spectrum of the given compound is Five. In question number, 5 given compound absence of proper substitution compound is achiral and so each type carbon in both rings resonance at same frequencies.

The answer to Question number-2 Stereochemistry: 13C NMR spectrum signal count

Note: In case Proper substitution means different substituent at the ortho position for chirality in biphenyl systems.

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