Organic Chemistry
Chapter 20 - Organic Chemistry
20.1 - Hydrocarbons
Organic Chemistry
The study of molecules containing C along w/ other elements (H, O, N, etc). Organic chemicals can either be synthetic/natural, harmless/harmful.
They can be represented in different ways
Alkanes [Saturated Hydrocarbons]
Consist of all single covalent bonds between C atoms, all C atoms in alkanes have sp3 hybrid orbitals which are bonded to four other atoms (C/H)
Since all alkanes: consist of C & H atoms, similar bonds, structures, & formulas; CnH2n+2
C chains have zigzag structure b/c of the bond angles being ~109.5º
Often drawn as a skeletal structure (line-angle structure) in which C atoms are not drawn
The base names come from the # of C:
Groups that come off the main chain (substituents) are named by changing the “ane” ending above to “yl”, for e.g. the -CH3 group is the methyl group. When the group has 3+ C, different shapes are possible.
Types of Carbons
Procedure in Naming Linear Alkanes*
Find the longest C chain [gives base name]
# the C in the longest chain to give any substituents the lowest # if same, alphabetical order
Names & give the # of each substituents
2+ substituents present [indicate w/ prefix i.e. di, tri]
List in alphabetical order, ignore prefixes, not iso
* Rules change when functional groups +’ed refer to section on alkynes
Nomenclature of Alkynes [Naming System]
Contains a C-C triple bond, has suffix “yne”
Procedure is similar to alkanes but triple bond must be a part of longest C chain used for the base name
Position of triple bond is indicated by # & should be as low as possible, true for all functional groups [structures w/ more than C & H atoms]
Nomenclature of Alkenes
Contains a double bond, has suffix “ene”
The procedure for naming alkenes is the same as that of alkynes
Double bonds do not rotate, results in geometric isomers [componds w/ same molecular formula & groups bonded to another but different spatial arrangements of these groups]
Cahn, Ingold & Prelog Rules
Used to identify which groups have priority in alkenes w/ many substituents
> Priority groups are on the same side of the double bond [Z isomer]
> Priority groups are on opposite sides of the double bond [E isomer]
> atomic # = > priority e.g. Br > Cl
If the groups start w/ the same atom, move out along the chain to find the first difference
Nomenclature of Chiral Molecules
If a molecule can be superimposed on its mirror image [achiral], if not [chiral]
Shortcut in 1110: look for a C w/ 4 different groups attached
Enantiomers
They have all the same physical properties (m.p, b.p, solubility, density) & react the same way w/ achiral chemicals
R & S Configurations
The two possible configs. At C are specified using Cahn, Ingold & Prelog rules
Priority is assigned to each of the groups & the lowest priority group is oriented away from the viewer
R config. (rectus/right) if arrow drawn from the highest priority to second is clockwise
S config. (sinister/left) if counterclockwise
20.2 & 20.3 - Alcohols, Ethers, etc.
Nomenclature of Other Functional Groups
Alcohols
Have a -OH group on a C
Suffix “ol”
Can form hydrogen bonds, thus have relatively high bps.
Aldehydes
Have a C-O double bond & have H off the C
Suffix “al” always ends a chain thus assigned to C1 1 not written
Ketones
Have a C-O double bond & two C off the C
Suffix “one” to indicate location of C-O double bond
Carboxylic Acids
Have a C-O double bond & an OH group off C
Suffix “oic acid”
Starts chain [linear carboxylic acids] thus 1 not written
Are weak acids & can hydrogen bond
Esters
Have a C-O double bond & an O w/ C attached to it
Suffix “oate”
The part attached to O is named like a substituent
Halogens
Contains: F, Cl, Br, I
Are named as substituents and the # of every halogen must be given in the name
Chapter 3 - Conformation and Stereochemistry
3.1 - Conformations of Open-Chain Organic Molecules
Stereochemistry
A branch of chemistry that studies the different spatial arrangements of atoms in a molecule
Cis/trans isomerism
Chair/boat conformations
Conformations and Conformational Isomer (Conformer)
Different spatial arrangements of atoms in a molecule result from σ bond rotation are called conformational isomers (conformer) in organic chemistry
Newman Projection
A convenient drawing convention is used to visualize different conformations of a molecule better
we look lengthwise down a specific bond of interest – in this case, the carbon-carbon bond in ethane. We depict the ‘front’ atom as a dot, and the ‘back’ atom as a larger circle
Bonds are NOT represented by dashed/wedges
Types of Isomerism
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Conformational
Different conformations of the same molecule result from rotation around a single bond
Staggered vs. Eclipsed Conformations
Staggered conformation is where the molecule is in the lowest energy conformation because the distance between the front and back C-H bonds are maximized giving it the most stable conformation. DA = 60º
Eclipsed conformation is where the molecule is in the highest energy conformation. DA = 0º
The dihedral angle [DA] refers to the distance between the front & back carbon (eg. the angle formed from the front C-H bond and the back one).
Notice that although they are staggered, the two methyl groups are not as far apart as they could be. Is called gauche conformation.
Anti conformation, where the two methyl groups are positioned opposite each other DA = 180º
Steric strain, which is the repulsive interaction caused by the two bulky methyl groups being forced too close together.
3.2 - Conformations of Cyclic Organic Molecules
Cyclic organic molecules = cyclo, many of which are aromatic, thus planar
[Cis] 2 substituents on the same ring are both pointing towards the same side of the ring
[Trans] Opposite sides
Ring structures in organic molecules are usually 5-6 membered. 3-4 membered rings are sometimes found in nature but are significantly > in energy, thus are unstable due to angle strain; the 4 bonds around the sp3 hybridized C are forced out of their preferred tetrahedral angles.