Chemistry (AS Level)·Foundations of organic chemistry · NSSCAS 4.1.2
Shapes of Organic Molecules & How Mechanisms Work
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Carbon builds everything from a single simple rule. In this lesson you'll explain why ethane is tetrahedral at about 109.5° with free rotation, why ethene is flat and trigonal planar at about 120° with a shorter, stronger double bond and no rotation, and how to predict the shape of related molecules. Then you'll pick up the mechanism toolkit — homolytic and heterolytic fission, curly arrows, free radicals, nucleophiles and electrophiles — so the mechanisms in later lessons read like plain English.
What you'll learn in this lesson
By the end you should be able to (NSSCAS Chemistry (AS Level) 4.1.2):
- Describe and explain the shape of, and bond angles in, the ethane molecule (tetrahedral carbons, ~109.5°, C–C single bond, free rotation) using electron-pair repulsion theory
- Describe and explain the shape of, and bond angles in, the ethene molecule (trigonal planar carbons, ~120°, C=C double bond, no rotation) using electron-pair repulsion theory, including that C=C is shorter and stronger than C–C
- Predict the shapes of, and bond angles in, other related molecules
- Explain homolytic and heterolytic fission, showing how bond breaking produces free radicals or ions
- State what curly arrows represent (double-headed = movement of an electron pair; single-headed = movement of one electron) and define free radicals, nucleophiles and electrophiles
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