6. The compound H2NC(CN)=C(CN)NH2 is a tetramer of HCN (hydrocyanic acid).
How many geometrical isomers are possible in this compound?
a. 1
b. 2
c. 4
d. 6
2 geometrical isomers are possible for H2NC(CN)=C(CN)NH2.
This compound, known as diaminomaleonitrile (DAMN), forms as a tetramer of HCN through polymerization, featuring a central C=C double bond with identical -NH2 and -CN substituents on each carbon.
Structure Analysis
The molecule follows the general formula R1R2C=CR3R4, where R1 = R2 = NH2 on one carbon and R3 = R4 = CN on the other. Rotation around the double bond is restricted, enabling cis-trans isomerism since substituents on each carbon differ (NH2 ≠ CN). The cis form (diaminomaleonitrile) has NH2 groups on the same side; the trans form (diaminofumaronitrile) has them opposite.
Option Evaluation
-
a. 1: Incorrect, as the differing substituents (NH2 vs CN) on each C=C carbon allow two distinct arrangements.
-
b. 2: Correct, matching the cis and trans geometrical isomers observed in HCN tetramers.
-
c. 4: Incorrect; no chiral centers exist (sp2 carbons have planarity), ruling out additional optical isomers.
-
d. 6: Incorrect; lacks multiple double bonds or rings for more configurations.
The compound H2NC(CN)=C(CN)NH2, a key HCN tetramer also called diaminomaleonitrile (DAMN), arises in prebiotic chemistry via HCN polymerization. This molecule sparks interest in geometrical isomers H2NC(CN)=C(CN)NH2 due to its rigid C=C double bond, vital for CSIR NET Life Sciences exam questions on stereochemistry.
Molecular Structure Breakdown
H2NC(CN)=C(CN)NH2 features a central ethene unit: one carbon bears H2N- and NC- groups, the other identical pairs. sp2 hybridization enforces planarity, restricting rotation and enabling cis-trans isomerism where NH2 groups align same-side (cis, DAMN) or opposite (trans, diaminofumaronitrile). No chiral centers form, limiting isomers to 2.
| Isomer Type | Configuration | Key Feature | Relevance |
|---|---|---|---|
| Cis (DAMN) | NH2 on same side | Stable in HCN polymers | Prebiotic synthesis |
| Trans | NH2 on opposite sides | Less common isomer | Polymer intermediates |
Why Only 2 Geometrical Isomers?
Standard alkene rules apply: two unique substituents per C=C carbon yield 2 isomers. Options beyond 2 fail—no extra double bonds, rings, or asymmetry exist. This matches experimental HCN oligomer studies confirming cis-trans pair.
Exam Relevance for CSIR NET
Geometrical isomers H2NC(CN)=C(CN)NH2 tests double-bond stereochemistry in organic/prebiotic contexts. Answer: b. 2, as cis-trans forms dominate without optical activity.
