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 sp 2 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, sp 2 hybridization Dipoles possible Alkenyl hydrogen is relatively acidic. 4.NMR – Alkenyl hydrogens and carbons appear at low field: 1 H (δ = ppm); 13 C (δ = ppm) J trans > J cis ; j geminal very small; J allylic variable, small. 5.IR – Measures vibration excitation 1-10 kcal/mol ( μm; 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 = (H sat – H actual )/2 H sat = 2n C + 2 – N X + N N (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).