important concepts11 1.alkenes – unsaturated molecules iupac names are derived from the longest...
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Important Concepts11
1. Alkenes – Unsaturated molecules• IUPAC names are derived from the longest chain
containing the double bond as the stem.• Double bond isomers: terminal, internal, cis, and
trans• Tri- and tetra-substituted alkenes are named
according to the E,Z system.
2. Double Bond – Consists of a bond and a bond• bond: overlap of two sp2 hybrid lobes on carbon• bond: overlap of two remaining p orbitals• bond E (~65 kcal/mol); bond E (~108 kcal/mol)
Important Concepts11
3. Alkene Properties – • Flat, sp2 hybridization• Dipoles possible• Alkenyl hydrogen is relatively acidic.
4. NMR –• Alkenyl hydrogens and carbons appear at low field:• 1H (δ = 4.6 - 5.7 ppm); 13C (δ = 100 -140 ppm)• Jtrans > Jcis; jgeminal very small; Jallylic variable, small.
5. IR – Measures vibration excitation• 1-10 kcal/mol (2.5-16.7 μm; 600-4000 cm-1)• Characteristic peaks for stretching, bending and
other vibrational modes• Fingerprint region (<1500 cm-1)
Important Concepts11
6. Alkane IR –• C-H Stretching: 2840 to 3000 cm-1
• C=C Stretching: 1620 to 1680 cm-1
• Alkenyl C-H Stretching: ~3100 cm-1
• Bending Modes: below 1500 cm-1
• Alcohols: Broad O-H stretch: between 3200 and 3650 cm-1
7. Degree of Unsaturation – Number of rings + number of bonds:
• Degree of unsaturation = (Hsat – Hactual)/2• Hsat = 2nC + 2 – NX + NN (disregard oxygen and
sulfur)
Important Concepts11
8. Heats of Hydrogenation – indicate relative stability of isomeric alkenes.
• Stability decreases with decreasing substitution.• Trans isomers are more stable than cis.
9. Eliminations of Haloalkanes (and other alkyl derivatives) –
• Follow the Sayzex rule (non-bulky base, internal alkene formation) or the Hofmann rule (bulky base, terminal alkene formation).
• Trans alkene products predominate over cis.• Elimination is stereospecific (dictated by the anti
transition state).
Important Concepts11
10. Dehydration of Alcohols – Dehydration in the presence of strong acid results in a mixture of products (major constituent is the most stable alkene).