Organics (Isomerism)

What is isomerism?

compounds with the same molecular formula, but different arrangements. Same number of each element, different names

What does isomerism consist of?

Structural isomerism, or stereoisomerism

Structural Isomerism

atoms and functional groups attached in different ways. Different bonding

Stereoisomerism

different spatial arrangements of atoms in molecules. The 3D arrangement

What does stereoisomerism consist of?

Configurational and conformation isomerism

Configurational isomerism

rigid and only made into isomers by breaking covalent bonds. No free rotation.

conformational isomerism

free rotation around sigma bonds, causing conformation changes

What does configuration isomerism consist of?

cis-trans and E/Z, restricted rotation around atoms

Optical isomerism, chirality of asymmetric carbon atom

How to determine if something is a primary, secondary, tertiary, or quaternary carbon?

usually defined based on how many carbons it’s attached to (primary is 1 carbon…), but can also be based on number of hydrogen. Primary is 3 or more hydrogens, secondary is 2 hydrogens, tertiary is 1 hydrogen, quaternary is no hydrogens

Note about methane

usually based on the definition where a carbon is made primary or secondary based on the number of carbons it’s bonded to doesn’t work for methane, so use the number of hydrogens. It has 3 or more, so its primary

Structural Isomers

same molecular formula, so same number of each atom, but different bonding and names

Geometric isomers

same molecular formula, but different groups on different sides because of restricted rotation

What type of bonding does geometric isomers have?

Geometric isomers only exist within double bonds, as single bonds have freee rotation, and triple bonds are too restricted

Examples of Geometric Isomers

cis/trans. Only works when substituents are the same

Cis the groups are on the same side, trans when the substituents are on opposite sides

What naming is used when the substituents are different?

E/Z. On the bottom of the double bond, look for the substituent with the greatest atomic number, Its given priority. Then look so the substituents above the bond and assign priority based on atomic number.

When priority groups are on the same side, its Z

When priority groups are on opposite sides its E

How to assign priority if the first atom on the same side of the double bond is the same?

Then look at the atomic number of the next atom. Atoms that are bonded to groups with greater atomic numbers have greater priority

What do geometric isomers usually share?

Similar chemical properties, but different physical properties. Some molecules are also large enough to have intra-molecule forces by hydrogen bonding within itself

Optical Isomer

Same molecular formula, but different orientation around stereocenter

What is a stereocenter and chiral center?

A stereocenter is a carbon that has 3 different groups attached to it.

A carbon is chiral when bonded to four different groups

What is an asymmetric center?

when a chiral carbon is bonded to four different groups, and when 2 molecules are mirrors of each each

How to assign R/S to optical isomers?

Rank four groups around chiral carbon, with the highest priority being 1…, then make the #4, or lowest priority group, point into page, then assign R and S

What are R and S?

R is clockwise, and right

S is counter-clockwise and left

What do optically active species do?

they rotate plane-polarized light.

Unpolarized light will pass thru a polarizer, which filters light, and then go into an optically active compound, which rotates it

What will happen when unpolarized light passes thru a polarizer and into optically active light?

The light becomes polarized and filtered and then rotates by the optically active light in the polarimeter tube

What comes after a polarimeter tube?

An analyzer, or filter then rotates light some more until visible.

What will happen in a optically inactive sample?

If the compounds are optically inactive, then the plane-polarized light won’t rotate

What is racemate?

Racemic mixture with equal quantities of both enantiomers

What are enantiomers?

opposite configurations at all chiral centers (nonsuperimposable, mirror images)

What are diastereomers?

opposite configurations at some chiral centers

Flipped. Some chiral centers are both R, but one could be S instead

What is saturated?

saturated only contains single bonds, and has the maximum subsituents

What are some characteristics of alkanes?

only contain single bonds, extremely unreactive, not very polar and high bond enthalpies

Need very high energy reactions, like combustion

Note about combustion coefficients

the things burning, the hydrocarbon, must always have a coefficient of 1, as it’s per

Free radical substitution

Halogenation with Cl2 or Br2 replacing hydrogen H with halogen X

Chain Mechanism

Initiation (homolytic fission), propagation, and termination

The Cl radical is used and then regenerated, like a catalyst.

The products of the hydrocarbon and halogen reaction are a HX and the hydrocarbon and X, the X being the halogen

Initiation

The initiation is the

What is homolytic fission?

Splits bond evenly producing two free radicals, with unpaired electrons. Draw the fish hooks

Propagation

The hydrocarbon breaks off a H, the H bonds to the halogen radical, and leaving a hydrocarbon radical.

The hydrocarbon radical splits bond between halogen, the hydrocarbon radical and

The Hydrogen with one less hydrogen then splits another bond to make free halogen radicals and the hydrocarbon bonds with a halogen

Termination

Halogen radicals combine to make diatomic halogen

Halogen radical and hydrocarbon radical combine

Hydrocarbon radicals combine, but only in trace amounts, because usually when hydrocarbon radicals for, combines with halogen radicals, or splits halogen bond

Fisher Projection

left and right sides are out of page, up and down are into page