Transcription of Newman Projections – More Practice- Answer Key
1 Newman Projections More Practice Answer Key I. For each of the following, draw the best (most stable) and worst (least stable). Newman projection , relative to the bond indicated in each question. The most stable conformations will be staggered conformations with the largest groups ANTI to each other. The least stable, highest energy conformation will have an eclipsed conformation with the largest groups sterically overlapped on top of each other. a. butane, C2-C3 bond H CH3 CH3. H H CH3. CH3 H. H 3C. H. H H H. HH H H. CH3. BEST- ANTI WORST- FULLY. ECLIPSED (steric and torsional strain energy). b. 1-chloropropane, C1-C2 bond H Cl CH3. H H Cl CH3 H. Cl H. H H H. HH H H. CH3. WORST- FULLY.
2 BEST- ANTI. ECLIPSED (steric and torsional strain energy). c. 2-methylbutane, C2-C3 bond CH3 CH3 CH3. CH3. CH3 H H CH3 CH3. H H H. CH3. H 3C. H H CH3 H C H. H 3 HH CH3H H HH. H 3C. CH3 CH3. equal in energy - mirror images equal in energy - mirror images BEST - only one Gauche WORST - one methyl eclipsed steric interaction with another methyl (steric AND torsional strain energy). d. 2,2-dimethylbutane, C2-C3 bond CH3 CH3. CH3 H. H 3C H CH3. CH3. H 3C. H H 3C CH3. H. HC. H CH3H. CH3 3. BEST - two Gauche steric WORST - eclipsed and always with a interactions (unavoidable in methyl-methyl steric overlap (steric any staggered situation) AND torsional strain energy). e. 2-chloro-2-methylpentane, C2-C3 bond (Cl is smaller than methyl).
3 CH3 CH2CH3 CH2CH3. CH3 CH3. Cl H H H CH3 CH3. H. CH2CH3. H 3C. H Cl H H 3C Cl CH3. HCl CH3H H. H 3C Cl CH2CH3 CH2CH3. equal in energy - mirror images equal in energy - mirror images BEST - lowest energy occurs when WORST - highest energy occurs when largest group (ethyl) is Gauche to largest group overlaps with either smallest group (Cl) and not both methyl group (bigger than Cl) (steric methyls (steric interaction) AND torsional strain energies). II. Rotation Barriers: Rank the rotational barriers relative to the highlighted bonds, with 1 having the highest barrier of rotation (assume that a chloride is smaller than a methyl group). Cl 6 5 4 1 2 3. The worst conformation is one that is totally eclipsed and the energy of the totally eclipsed conformation increases with increasing steric interactions.
4 Larger groups have more steric strain and this results in a larger barrier of rotation. Isopropyl groups (in #1 and #2) are larger than ethyl groups (in #1, # and #3), which are larger than methyl groups, etc. III. For each of the following, determine what strain energy is involved in each Newman projection (torsional and/or steric) to explain why the first Newman projection is more stable than the second. (-Cl is smaller than any alkyl group). a. Butane, C2-C3 (front carbon is C2) STERICS. CH3 CH3. H H H CH3 Steric interaction - one Gauche interaction H H H H. CH3 H. b. Butane, C2-C3 (front carbon is C2) TORSIONAL. CH3 H. CH3. H H. Torsional strain from eclipsing interaction (the first conformation is staggered).
5 H H H CH. HH 3. CH3. c. Butane, C2-C3 (front carbon is C2) TORSIONAL and STERICS. CH3. CH3 CH3. H H Torsional strain from eclipsing interaction (the first conformation is staggered and has no torsional strain). H and Steric strain from Methyl-Methyl overlap H HH H H. CH3. d. Ethane, C1-C2 (front carbon is C1) TORSIONAL. H H. H H. H. Torsional strain from eclipsing interaction (the first conformation is staggered). H H. HH H H. H. e. 2,3-dimethylpentane, C3-C4 (front carbon is C3) STERICS. CH3 CH3. H 3C H H H Larger steric strain from Isopropyl-Methyl Gauche interaction than the methyl-methyl overlap in the H CH(CH3)2 H CH(CH3)2 first conformation H CH3. f. 2,3-dimethylpentane, C3-C4 (front carbon is C4) STERICS (both have torsional strain so that doesn't make the second one less stable).
6 CH(CH3)2 CH(CH3)2. H CH3 Larger steric strain from Isopropyl-Methyl eclipsing interaction than the methyl-methyl overlap in the first conformation (both have torsional strain). CH3. HH HH H CH3. CH3. g. 2-methylbutane, C2-C3 (front carbon is C3) STERICS. CH3 CH3. H 3C H H CH3 Steric interaction - two Gauche interactions in the second conformation but only one in the first H H H CH3. CH3 H.
