Unit 3: Alkanes and Conformational Analysis
Molecules are never frozen: bonds spin, rings pucker, and a chair flip trades every axial substituent for an equatorial one.
8 topics · 30 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Nomenclature of Alkanes
- How it worksHow do you find the longest continuous chain and number it from the correct end to name a branched alkane?
- Worked exampleFind the correct IUPAC name for CH3CH(CH3)CH2CH2CH3.
- Common mistakeWhy isn't the longest chain drawn on the page always the parent chain you should name from?
- Going furtherWhat changes about a name when two different substituents are tied for giving the lowest locants?
Your turn1 of 2
What is the correct IUPAC name for CH3CH(CH3)CH2CH2CH3?Degrees of Unsaturation
- How it worksHow does comparing a formula against the fully saturated CnH2n+2 count reveal rings plus π bonds?
- Worked exampleFind the degrees of unsaturation in a compound with molecular formula C6H10.
- Worked exampleFind the degrees of unsaturation in C8H8O and decide whether it could contain a benzene ring.
Your turn1 of 2
A hydrocarbon's molecular formula has 14 carbons and 8 pairs of hydrogen atoms (that is, 8 times 2 hydrogens total, with no other elements present). How many degrees of unsaturation, rings plus π bonds combined, does it have?Newman Projections and Rotation About a Single Bond
- How it worksHow does a Newman projection turn a 3D molecule viewed down one bond into a front atom and a back atom?
- How it worksHow does the energy of ethane rise and fall as the back carbon rotates through eclipsed and staggered positions?
- Worked exampleFind the dihedral angle between two front hydrogens in the fully staggered Newman projection of ethane.
- Common mistakeWhy isn't the staggered conformation of ethane just an arbitrary drawing choice with no energy behind it?
Your turn1 of 2
In a Newman projection of ethane looking down the C-C bond, the three hydrogens on the front carbon are spaced evenly around it. What is the angle between any two adjacent front hydrogens as drawn in the 2D projection?Conformations of Butane and Gauche Strain
- How it worksHow do the two methyl groups in butane clash to create gauche strain in one staggered conformation?
- Worked exampleFind the four named conformations of butane in order of rising energy as the C2-C3 bond rotates a full 360 degrees.
- Going furtherWhat happens to butane's energy diagram if one methyl group is swapped for a bulkier tert-butyl group?
Your turn1 of 2
As the C2-C3 bond of butane rotates a full 360 degrees, four named conformations appear. Rank them from lowest to highest energy.Cyclohexane Chair Conformations
- How it worksHow does puckering into a chair let every carbon in cyclohexane reach a perfect 109.5 degree bond angle?
- Worked exampleWalk me through drawing a cyclohexane chair and labeling every axial and equatorial position around it.
- Common mistakeWhy isn't the flat hexagon drawing of cyclohexane an accurate picture of the molecule's real shape?
- Going furtherWhat happens to the ring's shape when cyclohexane is forced through the higher-energy boat conformation?
Your turn1 of 2
In a cyclohexane chair, each carbon has one axial and one equatorial position. Going around the ring from one carbon to the next, how do the axial positions alternate in direction?Axial vs Equatorial and the Chair Flip
- How it worksHow does flipping a cyclohexane chair turn every axial position into an equatorial one and back again?
- How it worksHow does a bulky substituent end up equatorial far more often than axial once the ring keeps flipping?
- Worked exampleFind which chair conformation of methylcyclohexane is lower in energy and by roughly how many kcal/mol.
- Common mistakeWhy doesn't the chair flip break or reform any of cyclohexane's carbon-carbon bonds?
- Going furtherWhat happens to the ring flip once a second substituent locks the more stable chair permanently in place?
Your turn1 of 2
A monosubstituted cyclohexane's equatorial chair is more stable than its axial chair by 2.24 kcal/mol at 298 K. Using the equilibrium constant K = edg / (0.001987·298), what percent of the molecules are in the equatorial conformation at equilibrium?1,3-Diaxial Interactions and A-Values
- How it worksHow do axial substituents on carbons 1, 3, and 5 end up close enough to strain each other on one face?
- Worked exampleFind the total 1,3-diaxial strain for an axial tert-butyl group using an A-value of about 4.9 kcal/mol.
- Going furtherWhat happens to a ring's preferred chair once a substituent gets too bulky to ever sit axial?
Your turn1 of 2
Each 1,3-diaxial interaction between an axial substituent and an axial hydrogen at C3 or C5 contributes about 1.97 kcal/mol of strain. Since an axial substituent interacts this way with both the axial hydrogen at C3 and the axial hydrogen at C5, what is its total 1,3-diaxial strain, in kcal/mol?Cis and Trans Disubstituted Cyclohexanes
- How it worksHow does the cis or trans relationship between two substituents lock in which positions they can share?
- Worked exampleFind whether the more stable chair of trans-1,2-dimethylcyclohexane has both methyls equatorial or one of each.
- Common mistakeWhy isn't cis-1,4-dimethylcyclohexane able to put both of its methyl groups equatorial at once?
- Ties togetherHow does a ring's cis or trans geometry relate to the R and S stereochemistry of its substituents?
Your turn1 of 2
In trans-1,2-dimethylcyclohexane, can both methyl groups be equatorial in the same chair conformation?