Orgo notes - Bredt\'s rule, elements of unsaturation, cycloalkenes PDF

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Bredt's rule, elements of unsaturation, cycloalkenes...

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Degrees of unsaturation  Structural element that decreases the number of hydrogens in the molecule by two  Also called index of hydrogen deficiency  Double bonds and rings are elements of unsaturation  Triple bonds = two pi bonds = two elements of unsaturation  Number of elements on unsaturation = (2C + 2 – H) / 2 o Doesn’t tell us what the elements are  For heteroatoms: number of elements of unsaturation = [2C + 2 + N – (H + Hal)] / 2 Bredt’s Rule  Bridged, bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms  Fused bicyclic compound can have double bond Cycloalkenes  Ring makes bigger difference only is there is ring strain, either because of small ring or because of trans double bond  Rings that are 5-membered of larger can easily accommodate double bonds, and these cycloalkenes react much like straight-chain alkenes  Stability of cycloalkene: o Cis more stable than trans in small cycloalkenes o Small rings have additional ring strain o Must have at least 8 carbons to form stable trans double bond (needed to close the loop) o For cyclodecene and larger, trans double bond almost as stable as cis o In linear alkenes, trans more stable because of sterics Nomenclature  Find longest continuous chain that includes double-bonded carbons  Ending -ane changes to -ene  Number chain so double bond has lowest possible number  In a ring, double bond assumed to be between carbon one and two  Include number of double bond in name (i.e. but-1-ene or but-2-ene)  Use di-, tri-, or tetra- before ending -ene to specify how many double bonds are present (i.e. cycloocta-1,3,5,7-tetraene)...