Ch. 4 - Isomerism 1

what are isomers

molecules with same formula that are different in some way

what are constitutional isomers?

isomers with different connectivities

what are stereoisomers?

isomers with same connectivity

what are conformers?

isomers with same connectivity that are arranged differently by rotations about single bonds

what are configurational isomers?

isomers with the same connectivity that are not different by rotations about single bonds

what are enantiomers?

isomers with same connectivity that are not different by rotations about single bonds that are non-superimposable mirror images of each other

what are diastereomers?

isomers with same connectivity that are not different by rotations about a single bond that do not have a mirror image

what is dihedral angle?

angle at which atoms on different carbons are at relative to one another in a Newman projection

what is torsional strain?

slight repulsion between electron clouds of adjacent C-H bonds in eclipsed conformers

are eclipsed or staggered conformers higher energy? why?

eclipsed because of strain

true or false: staggered conformers interconvert

true

what is anti in a Newman projection?

when bulky groups are 180 degrees apart

what is gauche Newman projection

bulky groups are 60 degrees apart

what is steric strain?

increase in E from electron repulsion caused by atoms/groups occupying the same physical space but are not actually bonded together

which conformation has all anti groups and thus is most stable?

zigzag

what are the two ways to represent conformers?

1) dash-wedge notation

2) newman projection

what causes ring strain?

geometric constraints of a ring (angle strain + steric/torsional strain)

what is angle strain?

increased E due to deviations in bond angles from VSEPR ideal bond angle

what is the difference between torsional and steric strain?

torsional strain arises from overlap of parallel bonds. steric strain arises from crowding of unbonded atoms. You can tell which is which by determining if the strain is caused by the dihedral angle or the size of the molecule. Torsional strain is due to dihedral angle, and steric strain is due the larger size of some atoms/groups

why are cycloalkanes >3 atoms not flat?

adopt nonplanar conformations to reduce ring strain

why doesn’t cyclohexane experience any strain?

because its bond angles are 109.5 degrees, and none of the bonds are eclipsed so there is no torsional strain

when performing a chair flip, substituents change sides of the ring’s plane. true or false

false, they stay on the same side, they just switch from equatorial to axial or vice versa

what is 1,3 diaxial strain?

strain that arises when C1 and C3 are too close together creating steric strain

true or false: monosubstituted cyclohexane is always more stable when substituent is equatorial

true

what are the steps for drawing the most stable chair conformation?

1) make the largest subst. equatorial

2) Draw other subst. relative to the first one (location and cis/trans)

true or false: cis and trans is independent of axial/equatorial position

true! two atoms can be trans and both be equatorial if they point in opposite directions

what is IHD?

index of hydrogen deficiency (AKA degree of unsaturation)

what is the formula for IHD when you have the structure of the molecule?

number of pi bonds + number of ringstru

true or false: not all constitutional isomers will have the same IHD

false; they have to because same molecular formula

what does it mean when a molecule is saturated?

it has the max # of H possible

what effect do halogens have on IHD?

the halogen replaces one H; # H’s decreases by one so we need to add the number of halogens to the number of H’s what

what effect does oxygen have on IHD?

it inserts itself between the C and the H so it has no net effect; we can ignore it when calculating IHD

what effect does nitrogen have on IHD?

N adds another H; # H’s increases by one, so we must subtract the # of N from the # of H when calculating

what is the formula for IHD when given the formula?

(2n + 2 - (#H + #X - #N))/2 = IHD