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:

CH21 – 11.19.13 , R.D. Bolinas Chapter 2: Alkanes

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

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 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. • 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. General Formula: (CH2)n Note: This limits the amount of rotational freedom usual C-C single bonds have!!

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)

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 orientations • WE HAVE RELATIVE FIXED POSITIONS • Sometimes noted as up/down with respect to a certain atom/group • Cis- (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 up down up down 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

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The chemical reactivity of organic molecules come from their FUNCTIONAL GROUPS