Transcript AP Ch. 25 Notes

Ch. 25: Organic Chemistry
• Organic chemistry is the branch of chemistry that studies carbon
compounds.
• Organic compounds were originally thought to be synthesized only in
living organisms until…
- Friedrich Wöhler synthesized urea (1828) to dispel the idea.
• Now many organic compounds are synthesized: synthetic oil,
gasoline, rubber, plastics…
General Characteristics of Organic Compounds
• Structure: Carbon can exhibit three different types of hybridization…
- sp3 hybridized carbons for tetrahedral geometries
- sp2 hybridized carbons for trigonal planar geometries
- sp hybridized carbons for linear geometries
Carbon’s Hybridization
Remember: Carbon’s hybridization relates to the # of atoms that it
is bonded to… 4 atoms= sp3
3 atoms= sp2
2 atoms= sp
The Stabilities of Organic Molecules
• Stability: Carbon also forms strong bonds with itself.
• Therefore, C can form stable long chain or ring structures.
cyclopropane
(unstable)
CnH(2n+2)
CnH(2n)
The Stabilities of Organic Molecules
• Remember:
- Bond strength increases from single to double to triple bond.
- Bond length decreases in the same direction.
- Strongest & shortest bond  C ≡ C
• Carbon and hydrogen have very similar electronegativities, so the
C-H bond is essentially non-polar.
• Therefore, compounds containing C-C and C-H bonds are stable.
• Adding certain groups of atoms called “functional groups”
introduces reactivity into organic molecules.
Solubility of Organic Compounds
• Solubility in water varies:
• Long nonpolar molecules are insoluble in water.
• Polar groups that are added will increase solubility.
Introduction to Organic Compounds
• The simplest type of organic compounds are the “alkanes”.
• Alkanes contain only single bonds.
• These compounds are referred to as “saturated hydrocarbons”…C is
allowed to bond to the maximum number of hydrogen atoms.
• Alkanes have the general formula: CnH(2n+2)
Isomers
• “Straight chain” hydrocarbons have each C atom joined in a
continuous chain.
• In a straight chain hydrocarbon no one C atom may be attached
to more than two other C atoms.
• Straight chain hydrocarbons are not linear!
• Each C atom is tetrahedral, so the chains are actually bent.
• Branched chain hydrocarbons are possible for four or more C
atoms, which give rise to “isomers”…same formula, different
properties
• Structural isomers are compounds having the same molecular
formula but differ in the bonding arrangement of the atoms.
- Example: There are 2 hydrocarbons with the formula C4H10.
Structural Isomers
Structural Isomers
Pentane (C5H12) has 3 structural isomers.
• The # of isomers greatly increases with the # of carbon atoms:
Octane, (C8H18), has 18 isomers and decane, (C10H22), has 75 isomers!
Stereoisomers
• Stereoisomers have the same formula and the same
bonding of atoms, but they differ in the spatial arrangement
of the atoms.
- Example: “Geometric Isomers”
Since the double bond cannot rotate, they are truly different!
Stereoisomers
• Optical isomers (or enantomers) are also stereoisomers.
They are molecules that are “nonsuperimposable mirror
images” of each other.
• They rotate plane polarized light differently.
• In order to have an optical isomer, you must have a “chiral
center”…a carbon atom with 4 different attached groups.
Optical Isomer
2-bromopentane
Optical Isomer
Enantiomers of alanine, an amino acid
Optical Isomers
• Here’s another example of optical isomers…
• Consider the compound carvone:
- The one on the left is responsible for the odor and
taste of caraway and dill.
- The one on the right is isolated from spearmint.
Unsaturated Hydrocarbons
• Alkenes: Contain a double bond…C = C.
• The double bond increases its reactivity. (Often times,
elements are added to the compound via the double bond
being broken.)
• The simplest alkenes are H2C=CH2 (ethene) and
CH3CH=CH2 (propene)…(their trivial names are ethylene
and propylene.)
• Alkenes are named in the same way as alkanes with the
suffix -ene replacing the -ane in alkanes.
• The location of the double bond is indicated by a number.
(We will get more into naming compounds later!)
Alkenes
Geometric isomers (“cis-trans”) only occur in compounds with a double bond.
Unsaturated Hydrocarbons
• Alkynes: Contain a triple bond…C ≡ C.
• The triple bond increases its reactivity even more vs. C = C.
• The simplest alkyne is HC≡CH (ethyne, or acetylene.)
• When it is burned in the presence of oxygen (oxyacetylene
torch) the temperature is about 5300º F!
• Alkynes are named in the same way as alkenes with the
suffix -yne replacing the -ene for alkenes.
Unsaturated Hydrocarbons
• Aromatic hydrocarbons: carbon atoms are connected in a
planar ring structure. (They have a distinct odor to them.)
- The carbons are linked by both σ and -bonds.
- The best known example is benzene (C6H6).
Aromatic Hydrocarbons
Benzene (C6H6).
The delocalized -bonds makes aromatic compounds rather stable.
Summary of Hydrocarbons
Naming Organic Compounds
• All organic molecule names have three parts:
- Prefix, which tells the nature of the substituents.
- Base, which gives the number of carbons.
- Suffix, which gives the family (alkanes, alkenes, etc.)
• Rules for naming compounds are given by the International Union for
Pure and Applied Chemistry (IUPAC).
• We will only do one example in these notes for practice, but we will
have more to do together later on!
Naming Organic Compounds
• Name this organic compound:
• Step 1: Find the longest chain and use it as the base name of the compound.
7 C’s
heptane
Naming Organic Compounds
• Step 2: Number the carbon atoms starting with the end closest to the
substituents.
- Groups attached to the main chain are called substituents.
- The preferred numbering will give substituents the lowest
numbers.
4
1
2
5
6
7
3
2- ________ 4- _________heptane
Naming Organic Compounds
• Step 3: Name and give the location of each substituent.
- When two or more substituents are present list them in
alphabetical order!
- A substituent group formed by removing an H atom from an
alkane is called an alkyl group.
• Alkyl groups are named by replacing the –ane ending with -yl.
Examples: CH4 is methane, and a –CH3 group is a methyl group.
–CH2CH3 would be an ethyl group.
–CH2CH2CH3 would be a propyl group.
• When there are two or more of the same substituent, the number of
that type of substituent is indicated by a prefix…
Examples: “dimethyl” indicates two methyl group substituents.
“dichloro” indicates two chlorine substituents.
Naming Organic Compounds
• Let’s name the substituents…
methyl-
4
1
2
5
6
7
3
ethyl-
Put them in alphabetical order!!!
4-ethyl-2-methylheptane
(Again, we will do more practice problems later.)
Functional Groups
• Hydrocarbons are relatively unreactive. For an organic
molecule to be reactive it needs something additional…
• A site of “reactivity” in an organic molecule is called a
functional group. Functional groups control how a molecule
functions.
• These groups determine the chemistry of the molecule.
• Functional group containing molecules can either be
saturated… (single bonds only-- alcohols, ethers, amines,
etc.) or unsaturated… (double bonds involved--carboxylic
acids, esters, amides, etc.)
(We usually use R to represent alkyl groups… methyl, ethyl, etc.)
Functional Groups
Functional Groups
Properties of Functional Groups
Alcohols (R-OH): do not liberate OH− ions in solution like bases
Examples: CH3OH, methanol, is used as a gasoline additive and a
fuel.
CH3CH2OH, ethyl alcohol, or ethanol, is “booze”.
Properties of Functional Groups
Ethers (R-O-R′): used as anesthetics, solvent for gums, fats, and wax.
Example: diethyl ether (CH3CH2 – O – CH2CH3)
O
Aldehydes:
R
H
Example: methanal (or formaldehyde)
(CH2=O) is used as a preservative for
biological specimens…it coagulates
proteins.
O
Ketones:
R
R'
Examples: fingernail polish remover is
propanone [or acetone… (CH3)2C=O ]
used as solvents for plastics; manufacture
of cordite, an explosive; carvone, (the
optical isomer example) is a ketone.
Properties of Functional Groups
Carboxylic Acids (COOH): found in fruits… (ascorbic acid, or
vitamin C; malic acid is in apples);
Acetic acid (CH3COOH) is vinegar;
O
Formic acid (HCOOH) was first obtained
by distillation of red ants (responsible for
R
OH
the sting in ant bites.)
Carboxylic acids are very weak acids/weak electrolytes.
Properties of Functional Groups
Esters (RCOOR’):
O
R
often have distinctive odors: ethyl
butyrate (C3H7COOC2H5) has the odor of
pineapple; butyl acetate (CH3COOC4H9)
has the odor of bananas.
OR'
Other Examples:
benzocaine (in sun burn lotions);
ethyl acetate (nail polish remover);
vegetable oils;
polyester thread;
aspirin…which has an ester functional group as well as a
carboxylic acid functional group (see previous slide)
Properties of Functional Groups
Amines and Amides (R3N):
• Amines are organic bases where R may be H or a hydrocarbon group.
• Just as alcohols, (R – OH), can be thought of organic forms of water,
(H – OH) amines can be thought of organic forms of ammonia, NH3.
Example: ethylamine, (CH3CH2NH2)
• Amines are among the most noxious-smelling of all organic
compounds.
• Amides are composites of carbonyl and amine functional groups:
carbonyl group
R
O
C
N R'
H
amine group
•Amides are the key functional group for amino acids which link up to
make proteins.
Functional Groups
Practice Problem:
What functional groups do
you see highlighted in
these sugars?
Answers:
* Glucose is an aldehyde.
* Fructose is a ketone.
* Both contain multiple
hydroxyl (or alcohol)
functional groups too.