The chemical reactivity of organic molecules come from their FUNCTIONAL GROUPS

The chemical reactivity of organic molecules come from their FUNCTIONAL GROUPS

Regardless of size!

Carbon-hydrogen backbones can stand alone…

…or they can combine with other elements like O,N,S,Cl,Br,F, etc…

… and each combination will have distinct physi-chemical properties.

Solubility (polar? Nonpolar?) Reactivity (pi bonds? Steric

hindrance? Etc..) Stable in acids? Bases? Reactive

with…? Electronegative-electropositive

regions:

Chapter 2: AlkanesCH21 – 11.19.13 , R.D. Bolinas

We call them: ALKANES

The simplest of all are the pure hydrocarbons with nothing but non-polar C-C, and C-H single bonds.

Alkanes are all C-C and C-H. Alkanes are purely singly bonded species all C’s are sp3 having the maximum carbon-to-hydrogen

ratio, THUS CALLING IT SATURATED CnH2n+2 = basic formula Highly nonpolar, trends in properties

related to: chain length, and “branching”

The C-C and C-H ‘lego’ can combine in many ways forming isomers

CONSTITUTIONAL ISOMERS = same constituents (e.g. no. of

carbons, hydrogens, etc.) but just different bonds.

We follow basic conventions to name and label them…

For straight chains: (n- means normal) (1) Meth < (2) Eth < (3) Prop < (4) But < penta… + ‘ane’ = (methane, n-butane…)

We can replace –ane with –yl to indicate it being only a side group…

And for the trickier cases with branching, we still follow standard rules.

And for the trickier cases with branching, we still follow standard rules.

And for the trickier cases with branching, we still follow standard rules.

And for the trickier cases with branching, we still follow standard rules.

And some non-systematic names are to be memorized…

To simplify representation, we also have conventions for drawing them.

We also have drawing conventions to show a more 3-D approach…

One important facet of a C-C single bond is that it can freely rotate…

The head-on σ-bonds are symmetrical and are the same regardless of rotational alignment

We simply need to have enough energy to overcome the torsional strain RESISTANCE TO BOND TWISTING leading to eclipsed bonds

One important facet of a C-C single bond is that it can freely rotate…

In one sense, we can account for this with energy costs of having bulky groups electron shells repel one another

Using our projections, we can analyze the probability of a 3-D form.

Using our projections, we can analyze the probability of a 3-D form.

Using our projections, we can analyze the probability of a 3-D form.

Using our projections, we can analyze the probability of a 3-D form.

Using our projections, we can analyze the probability of a 3-D form.

We draw the zig-zag form simply because ALL adjacent bonds are ANTI-STAGGERED!

MOST STABLE = MOST PROBABLE

Alkanes can also exist in rings called cycloalkanes.

Note: This limits the amount of rotational freedom usual C-C single bonds have!!

General Formula: (CH2)n

They similarly follow a systematic naming scheme…

They similarly follow a systematic naming scheme…

They similarly follow a systematic naming scheme…

Try naming this!

They similarly follow a systematic naming scheme…

Now, because of carbon’s sp3 hybridization, we don’t quite have a perfectly flat shape!

The 109.5° angle of a tetrahedral sp3 carbon will have to “compromise” somewhere near the angle of the polygon.

Now, because of carbon’s sp3 hybridization, we don’t quite have a perfectly flat shape!

We need to fight ANGLE STRAIN the strain due to the expansion or

contraction of bond angles

Now, because of carbon’s sp3 hybridization, we don’t quite have a perfectly flat shape!

The solution: 3D structures (if possible)

Now, because of carbon’s sp3 hybridization, we don’t quite have a perfectly flat shape!

The solution: 3D structures (if possible)

Now, because of carbon’s sp3 hybridization, we don’t quite have a perfectly flat shape!

The solution: 3D structures (if possible)

We have another kind of isomer- stereoisomers: different 3D orientations Remember that our carbons are all

sp3 and so they still retain more or less their 3D tetrahedral shape,

but unlike open chains, rotation is next to impossible.

WE HAVE RELATIVE FIXED POSITIONS.

We have another kind of isomer- stereoisomers: different 3D orientationsWE HAVE RELATIVE FIXED

POSITIONS Sometimes noted as up/down with

respect to a certain atom/groupCis- (same side) or trans-

(opposite) added to name

Cyclohexane is one of the most important cyclic systems we study…

It has 2 main configurations: the more stable CHAIR FORM and also a BOAT FORM

ANTI-STAGGERED

CHAIR form = has a backrest and a footrest, like a massage chair

Cyclohexane is one of the most important cyclic systems we study…

It has 2 main configurations: the more stable CHAIR FORM and also a BOAT FORM

BOAT / twist-boat

More details on the CHAIR CONFORMATION. We have bonds along the AXIS

(axial) We also have bonds spreading out of

the EQUATOR (equatorial)axial

equatorial

Stereochemistry for chair cyclohexane systems… 1. We need to notify “UP” and

“DOWN” positions for each substituent per carbon.

2. Opposite names (UP-DOWN) means trans-

up

downup

down

up

down

Since C-C bonds can rotate, we have a system that can ‘flip’.

Steric strain and charge repulsions affect its conformation.

Groups along the axial positions can block and repel each other (steric strain)

Steric strain and charge repulsions affect its conformation.

By flipping, we can move them to the equatorial position where they point outwards from each other.

MORE FAVORED Bulkier groups along equatorial

PROBLEM SOLVING

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