5.12 Handout #4

Handout #4, 5.12 Spring 2003, 2/14/02 Alkanes: Nomenclature and Conformational Analysis Name n-Alkane Formula (CnH2n+2) Name n-Alkyl Substituent # Constitutional Isomers CH4 C2H6 methane ethane –CH3 (–Me) –C2H5 (–Et) methyl ethyl 1 1 C3H8 C4H10 propane butane –C3H7 (–nPr) –C4H9 (–nBu) propyl butyl 1 2 C5H12 C6H14 pentane hexane –C5H11 –C6H13 pentyl hexyl 3 5 C7H16 C8H18 heptane octane –C7H15 –C8H17 heptyl octyl C9H20 C10H22 nonane decane –C9H19 –C10H21 nonyl decyl C11H24 C12H26 undecane dodecane –C11H23 –C12H25 undecyl dodecyl C13H28 C14H30 tridecane tetradecane –C13H27 –C14H29 tridecyl tetradecyl C15H32 pentadecane –C15H31 pentadecyl 9 18 etc. Constitutional Isomers (Be able to recognize these common fragments.) methylMe CH3 CH3 2 1 CH3 sec-butyl­ (1-methylpropyl-) s Bu 1 2 CH3 3 Pr iso-butyl­ (2-methylpropyl-) n-butyl­ nBu CH3 tert-butyl­ (1,1-dimethylethyl-) CH3 H3C tBu CH3 n CH3 Et iso-propyl­ (1-methylethyl-) iPr n-propyl­ ethyl­ CH3 1 CH3 2 sBu 1 2 CH3 3 CH3 neopentyl­ (2,2-dimethylpropyl) 1 3 2 CH3 H3C CH3 1 Conformational Analysis: The Fundamentals Newman Projections H H H H H H H H H H H back carbon (circle) H front carbon (point) θ = dihedral angle Barrier to Rotation: torsional energy; energy required to twist a bond into a specific conformation; Barrier to Rotation = Erel (highest energy conformer) – Erel (lowest energy conformer) Ethane H H H H H H staggered θ = 60° Erel = 0 kcal/mol Propane HH H H H H H H H Me HH H H H H staggered θ = 60° Erel = 0 kcal/mol eclipsed θ = 0° Erel = 3.0 kcal/mol Barrier to Rotation = 3.0 – 0 = 3.0 kcal/mol Me H eclipsed θ = 0° Erel = 3.3 kcal/mol Barrier to Rotation = 3.3 – 0 = 3.3 kcal/mol H/H-eclipsing = 1.0 kcal/mol H/Me-eclipsing = 1.3 kcal/mol Butane MeMe H H H H H fully eclipsed (eclipsed) θMe/Me = 0° E rel = 6.0 kcal/mol H Me H HMe Me H H H gauche (staggered) θMe/Me = 60° Erel = 0.9 kcal/mol H Me H H eclipsed (eclipsed) θMe/Me = 120° E rel = 3.6 kcal/mol Me Me H H anti (staggered) θ Me/Me = 180° Erel = 0 kcal/mol Barrier to Rotation = 6.0 – 0 = 6.0 kcal/mol Me/Me-eclipsing = 4.0 kcal/mol* Gauche Butane Interaction (gbi) = 0.9 kcal/mol Numbers to Remember H/H-eclipsing = 1.0 kcal/mol H/Me-eclipsing = 1.3 kcal/mol * In Wade, a Me/Me-eclipsing interaction is said to be worth 3.0 kcal/mol. Either value will be acceptable in this class, but most other sources assign the value of 4.0 kcal/mol. A Me/Me-eclipsing interaction is significantly higher than an H/H- or H/Me-eclipsing interaction because the hydrogens on the two methyl groups can actually bump into each other (steric strain). Me/Me-eclipsing = 4.0 kcal/mol* Gauche Butane (gbi) = 0.9 kcal/mol H H H H/H H H H H H H/Me H H H H Me/Me 2 Cycloalkanes: Names and Ring Strain cyclopropane (27.6 kcal/mol) cyclobutane (26.4 kcal/mol) cyclopentane (6.5 kcal/mol) cyclohexane no ring strain! cycloheptane (6.3 kcal/mol) cyclooctane (9.6 kcal/mol) Substituted Cyclohexane Drawing Cyclohexane Chairs Steps 1–3: Draw three pairs of parallel lines as shown X equatorial: C–X bond anti to ring bonds axial: C–X bond gauche to two ring bonds (1) (2) (3) Steps 4–6: Draw equatorial bonds parallel to ring bonds in bold X H (4) (5) (6) Steps 7,8: Draw axial bonds as vertical lines below and above ring. H • This means that axial groups experience 1,3-diaxial interactions; "bump into" other axial groups. X • EQUATORIAL PREFERRED A-Values to Remember –X (7) (8) Ring Flip 6 4 2 5 2 3 6 4 1 5 3 1 • Interconverts equatorial and axial substituents. • Energetic barrier = 10 kcal/mol; occurs rapidly at room temperature. –CH3 –CH2CH3 –CH(CH3)2 –C(CH3)3 –CN –OH –Cl –Br A-Value 1.74 1.8 2.1 5.4 0.2 1.0 0.5 0.6 • A-Value = magnitude of equatorial preference 3 Di-Substituted Cyclohexane: Which Conformer is Preferred? 1. Try to place all substituents equatorial. Me Me Me Me 1 3 Me 1 Me 3 Me 12 Me Me 1 Me 2 2 1 3 preferred Me 1 Me preferred 2. If you can't, place the substituent with the largest A-value equatorial. iPr Et iPr Et 1 1 1 3 Me Me 3 Et 3 Me 1 preferred 4 Et 1 Et Et 4 iPr 4 1 preferred REMEMBER: Midterm #1 is Friday, February 21, in Walker. It will cover reading and lecture material through Tuesday, February 18. This means that not all of the material on this handout will be covered on the exam. 4