isomers chapter 4

isomers

Different compounds
that have the same molecular
formula.

constitutional isomers

Isomers that have the same
molecular formula but
different connectivity – their
atoms are connected in a
different order.

conformational isomers ( conformers )

isomers that have the same connectivity but differ ONLY BY rotations about single bonds.

Newman projections

look at the image from left to right, the first carbon chain that appears is on top, and the last is on the back 

carbons bonded together are not visible in newman projections

dihedral angle

angle of rotation corresponding between the C-x or y bonds as they appear in the newman projections, basically the bonds that are not hydrogen 

conformational analysis

the plot of a molecules energy as a function of that bonds dihedral angle 

such as eclipsed conformation - where its kind of an off angle, or staggered where they are at equal distances 

which is more stable, staggered or eclipsed molecules

staggered are more stable because electron repulsion is at a minimum, 

what is torsional strain? which type of molecules ( staggered or eclipsed ) have higher torsional strain?

torsional strain is an increase in energy that arises in an eclipsed conformation

ring strain

increase in energy due to geometric contrainst on a ring structure

angle strain and ideal angle for hybridized Carbon rings

makes a significant contribution to ring strain because the ideal bond angle for a sp³ hybridized C is 109.5 degrees

steric strain

occurs when atoms or groups of atoms are in the same space

Describe how cyclobutane looks

slightly puckered with interior angles of 88 degrees, Basically a square that is slightly bent. see image.

describe how cyclopentane looks

envelope conformation that is slightly puckerd at the end. Bonds range from 102 to 106 degrees 

describe how cyclohexane looks

almost no ring strain, looks like a chair formation

ring strain is calculated from what? How?

heat of combustion → ring strain energy =[ (observed heat of combustion)] - (expected heat of combustion) 

describe axial and equatorial hydrogens

equatorial bonds line in the plane that is defined by the ring

axial bonds are perpendicular to the plane 

SEE IMAGE - red shows axial bonds, and green shows equatorial bonds

describe cis and trans isomers

differ by physical properties and stability

• Physical properties differ:

  • Cis often has a higher boiling point (more polarity).

  • Trans often has a higher melting point (more symmetry).

• Stability differs:

  • Trans is usually more stable than cis (less steric strain).

In short:

• Cis = same side

• Trans = opposite sides

see example of cis vs trans isomer in haworth projections

image example

explain the difference between diequatorial vs diaxial

diequatorial is much more stable that diaxial, ,

equatorial - axial and axial -equatorial  are moderately stable 

constitutional isomers ( aka structural isomers)

share the same molecular formula but differ in their connectivity

what are the rules for identifying constiuttional isomers

1. For each molecule, identify the parent chain or ring. This is the longest
continuous chain or ring of carbons that contains any C=C double and C≡C triple
bonds.
2. Number the carbons in the chain or ring. Make sure that:
• The carbon atoms involved in the double or triple bonds receive the lowest numbers,
or, if there are no such multiple bonds, the first substituent is attached to the lowest-
numbered carbon.
3. Establish the relative connectivity along the parent chain or ring. The
molecules must have different connectivities, and be constitutional isomers, if
they differ in:
• The size of the parent chain or ring.
• The numbers assigned to the carbons involved in the multiple bonds.
• The numbers assigned to the carbons to which any substituent is attached.
• The identities of the substituents attached to the same-numbered carbon.
• Otherwise, the molecules have the same connectivity and are not constitutional
isomers

explain how the melting point and boiling point are affected by the constitutional isomers’ structure

melting point is more affected by molecular symetry, the more symetrical it is, the higher the melting point,

boiling point is more affected by surface area _> longer chains have higher LDF’s → higher boiling points

What is Index of hydrogen deficiency and how to determine it

Give the formula for it.

Each double bond contributes 1 to a molecule’s IHD.
Each triple bond contributes 2 to a molecule’s IHD.
* Each ring contributes 1 to a molecule’s IHD

IHD =( max # of H’s - Actual # of H’s)/2

or better represented by ( ( 2*{carbons} +2 - (number of hydrogens) + (number of nitrogens ) - Number of hallogens))/2.

describe how to draw isomers

1. deterine the IHD, deterines the number of double or tripple bonds required as well as rings

2. draw all possible unique structures without H’s or halogens

3. add double bonds or tripple bonds to achieve the total IHD

4. add halogen atoms at various location to generate as many unique connectivities possible