Organic and Biological Chemistry
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Chapter 25 Organic and
Biological Chemistry
Chemistry, The Central Science, 11th edition Theodore L. Brown; H. Eugene LeMay, Jr.;
and Bruce E. Bursten
John D. Bookstaver St. Charles Community College
Cottleville, MO
Organic and Biological Chemistry
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Organic Chemistry • Organic chemistry is the chemistry
of carbon compounds. • Carbon has the ability to form long
chains. • Without this property, large
biomolecules such as proteins, lipids, carbohydrates, and nucleic acids could not form.
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Structure of Carbon Compounds
• There are three hybridization states and geometries found in organic compounds: – sp3 Tetrahedral – sp2 Trigonal planar – sp Linear
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Hydrocarbons
• There are four basic types of hydrocarbons: – Alkanes – Alkenes – Alkynes – Aromatic hydrocarbons
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Alkanes
• Alkanes contain only single bonds. • They are also known as saturated hydrocarbons.
– They are “saturated” with hydrogens.
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Formulas
• Lewis structures of alkanes look like this. • They are also called structural formulas. • They are often not convenient, though…
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Formulas
…so more often condensed formulas are used.
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Properties of Alkanes
• The only van der Waals force is the London dispersion force.
• The boiling point increases with the length of the chain.
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Structure of Alkanes
• Carbons in alkanes are sp3 hybrids. • They have a tetrahedral geometry and 109.5°
bond angles.
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Structure of Alkanes
• There are only σ-bonds in alkanes.
• There is free rotation about the C—C bonds.
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Isomers
Isomers have the same molecular formulas, but the atoms are bonded in a different order.
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Organic Nomenclature • There are three parts to a compound name:
– Base: This tells how many carbons are in the longest continuous chain.
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Organic Nomenclature • There are three parts to a compound name:
– Base: This tells how many carbons are in the longest continuous chain.
– Suffix: This tells what type of compound it is.
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Organic Nomenclature • There are three parts to a compound name:
– Base: This tells how many carbons are in the longest continuous chain.
– Suffix: This tells what type of compound it is. – Prefix: This tells what groups are attached to the
chain.
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How to Name a Compound
1. Find the longest chain in the molecule.
2. Number the chain from the end nearest the first substituent encountered.
3. List the substituents as a prefix along with the number(s) of the carbon(s) to which they are attached.
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How to Name a Compound
If there is more than one type of substituent in the molecule, list them alphabetically.
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Cycloalkanes
• Carbon can also form ringed structures. • Five- and six-membered rings are most stable.
– They can take on conformations in which their bond angles are very close to the tetrahedral angle.
– Smaller rings are quite strained.
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Reactions of Alkanes
• Alkanes are rather unreactive due to the presence of only C—C and C—H σ-bonds.
• Therefore, they make great nonpolar solvents.
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Alkenes
• Alkenes contain at least one carbon–carbon double bond.
• They are unsaturated. – That is, they have fewer than the maximum number of
hydrogens.
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Structure of Alkenes
• Unlike alkanes, alkenes cannot rotate freely about the double bond. – The side-to-side overlap in the π-bond makes this
impossible without breaking the π-bond.
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Structure of Alkenes
This creates geometric isomers, which differ from each other in the spatial arrangement of groups about the double bond.
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Properties of Alkenes
Structure also affects the physical properties of alkenes.
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Nomenclature of Alkenes • The chain is numbered so the double bond gets the
smallest possible number. • cis-Alkenes have the carbons in the chain on the
same side of the molecule. • trans-Alkenes have the carbons in the chain on
opposite sides of the molecule.
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Reactions of Alkenes
• One reaction of alkenes is the addition reaction. – In it, two atoms (e.g., bromine) add across the
double bond. – One π-bond and one σ-bond are replaced by two σ-bonds; therefore, ΔH is negative.
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Mechanism of Addition Reactions
• It is a two-step mechanism: – The first step is the slow, rate-determining
step. – The second step is fast.
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Mechanism of Addition Reactions
In the first step, the π-bond breaks and the new C—H bond and a cation form.
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Mechanism of Addition Reactions
In the second step, a new bond forms between the negative bromide ion and the positive carbon.
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Alkynes
• Alkynes contain at least one carbon–carbon triple bond.
• The carbons in the triple bond are sp-hybridized and have a linear geometry.
• They are also unsaturated.
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Nomenclature of Alkynes
• The method for naming alkynes is analogous to the naming of alkenes.
• However, the suffix is -yne rather than -ene.
4-methyl-2-pentyne
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Reactions of Alkynes
• Alkynes undergo many of the same reactions alkenes do.
• As with alkenes, the impetus for reaction is the replacement of π-bonds with σ-bonds.
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Aromatic Hydrocarbons
• Aromatic hydrocarbons are cyclic hydrocarbons that have some particular features.
• There is a p-orbital on each atom. – The molecule is planar.
• There is an odd number of electron pairs in the π-system.
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Aromatic Nomenclature
Many aromatic hydrocarbons are known by their common names.
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Reactions of Aromatic Compounds • In aromatic
compounds, unlike in alkenes and alkynes, each pair of π-electrons does not sit between two atoms.
• Rather, the electrons are delocalized; this stabilizes aromatic compounds.
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Reactions of Aromatic Compounds
• Due to this stabilization, aromatic compounds do not undergo addition reactions; they undergo substitution.
• In substitution reactions, hydrogen is replaced by a substituent.
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Structure of Aromatic Compounds
• Two substituents on a benzene ring could have three possible relationships: – ortho-: On adjacent carbons. – meta-: With one carbon between them. – para-: On opposite sides of ring.
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Reactions of Aromatic Compounds
Reactions of aromatic compounds often require a catalyst.
Halogenation
Friedel-Crafts Reaction
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Functional Groups
The term functional group is used to refer to parts of organic molecules where reactions tend to occur.
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Alcohols • Alcohols contain one or more hydroxyl groups,
—OH. • They are named
from the parent hydrocarbon; the suffix is changed to -ol and a number designates the carbon to which the hydroxyl is attached.
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Alcohols
• Alcohols are much more acidic than hydrocarbons. – pKa ~15 for most
alcohols. – Aromatic alcohols
have pKa ~10.
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Ethers
• Ethers tend to be quite unreactive. • Therefore, they are good polar solvents.
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Carbonyl Compounds
• The carbonyl group is a carbon-oxygen double bond.
• Carbonyl compounds include many classes of compounds.
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Aldehydes In an aldehyde, at least one hydrogen is attached to the carbonyl carbon.
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Ketones
In ketones, there are two carbons bonded to the carbonyl carbon.
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Carboxylic Acids
• Acids have a hydroxyl group bonded to the carbonyl group.
• They are tart tasting. • Carboxylic acids are
weak acids.
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Esters
• Esters are the products of reactions between carboxylic acids and alcohols.
• They are found in many fruits and perfumes.
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Amides
Amides are formed by the reaction of carboxylic acids with amines.
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Amines
• Amines are organic bases. • They generally have strong, unpleasant
odors.
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Chirality • Carbons with four different groups attached to
them are handed, or chiral. • These are optical isomers or stereoisomers. • If one stereoisomer is “right-handed,” its
enantiomer is “left-handed.”
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Chirality
• Many pharmaceuticals are chiral. • Often only one enantiomer is clinically
active.
S-ibuprofen
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Amino Acids and Proteins
• Proteins are polymers of α-amino acids.
• A condensation reaction between the amine end of one amino acid and the acid end of another produces a peptide bond.
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Amino Acids and Proteins
• Hydrogen bonding in peptide chains causes coils and helices in the chain.
• Kinking and folding of the coiled chain gives proteins a characteristic shape.
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Amino Acids and Proteins
• Most enzymes are proteins.
• The shape of the active site complements the shape of the substrate on which the enzyme acts; hence, the “lock- and-key” model.
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Carbohydrates
Simple sugars are polyhydroxy aldehydes or ketones.
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Carbohydrates
• In solution, they form cyclic structures.
• These can form chains of sugars that form structural molecules such as starch and cellulose.
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Nucleic Acids
Two of the building blocks of RNA and DNA are sugars (ribose or deoxyribose) and cyclic bases (adenine, guanine, cytosine, and thymine or uracil).
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Nucleic Acids
These combine with a phosphate to form a nucleotide.
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Nucleic Acids
Nucleotides combine to form the familiar double-helix form of the nucleic acids.