Download Organic Chemistry 25.2 Introduction to Hydrocarbons

Chapter 25 The Chemistry of Life: Organic Chemistry
25.1 Some General Characteristics of Organic Molecules
Organic chemistry is
The structures of organic molecules
The shapes of organic and biochemical molecules are important in determining
their physical and chemical properties. Consider carbon:
The stabilities of organic substances
The stability of organic substances varies – e.g. benzene has special stability due
to the delocalization of ! electrons.
Solubility and acid-base properties of organic
substances
The prevalence of C-C and C-H bonds results
in low overall polarity for many organic molecules,
and such molecules are soluble in nonpolar
solvents.
25.2 Introduction to Hydrocarbons
The simplest class of organic molecules is the hydrocarbons
25.3 Alkanes
Single bonds only; “saturated” hydrocarbons (saturated with hydrogen).
Structures of Alkanes
VSEPR theory predicts each
C atom is tetrahedral:
Structural Isomers
In straight-chain hydrocarbons the C atoms are joined in a continuous chain.
Branched-chain hydrocarbons are possible for alkanes with four or more C atoms.
Structures with different branches can be written for the same formula:
Structural isomers are compounds with the same molecular formula but different
bonding arrangements.
Nomenclature of Alkanes
Organic compounds are named according to rules established by the International
Union for Pure and Applied Chemistry (IUPAC).
prefix
To name a compound…
2. Number the chain from the end
nearest the first substituent
encountered
4. When two or more substituents are
present, list them in alphabetical order.
base
suffix
Cycloalkanes
Carbon can also form ringed structures. 5- and 6-membered rings are most stable:
Reactions of alkanes
Rather unreactive due to presence of only C–C and C–H "-bonds.
25.4 Unsaturated Hydrocarbons
Alkenes
Alkenes are unsaturated hydrocarbons that contain C and H atoms and at least
one C-C double bond.
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 prefix
Alkenes cannot rotate freely about the double bond (! bond = side-to-side overlap)
No rotation means
geometric isomers
are possible in alkenes
Structure affects physical properties of alkenes. Cis and trans do not interconvert.
Alkynes
Unsaturated hydrocarbons that contain at least one carbon–carbon triple bond.
Naming is analogous to alkenes
Addition Reactions of Alkenes and Alkynes
The dominant reactions for alkenes and alkynes are addition reactions:
Common additions include H2 (hydrogenation), H2O or hydrogen halides. Alkynes
are also capable of addition reactions:
Mechanism of Addition Reactions
A mechanism is an explanation of how a reaction proceeds.
Aromatic Hydrocarbons
Aromatic structures are formally related to
benzene (C6H6):
Highly unsaturated: very reactive?
Even though they contain ! bonds, aromatic
hydrocarbons undergo substitution reactions
more readily than addition reactions.
25.5 Functional Groups: Alcohols and Ethers
Hydrocarbons are relatively unreactive; for an organic molecule to be reactive it
needs something additional.
Alcohols, R–
R–OH
Named from parent hydrocarbon; suffix changed to -ol
The O–H bond is polar and can participate in hydrogen bonding, so alcohols are
more water soluble than alkanes.
Ethers, R–
R–O–R’
Compounds in which two hydrocarbons are linked by an oxygen are called ethers.
25.6 Compounds with a Carbonyl Group
The carbonyl group, C=O, plus the type of atoms attached to the carbonyl carbon
defines the particular kind of compound.
Aldehydes and Ketones
Aldehydes must have at least one H atom attached to the carbonyl C
Ketones have two carbons bonded to carbonyl carbon
Carboxylic Acids
Carboxylic acids contain a carbonyl group with an –OH attached
Esters
Esters can be prepared by condensation reactions involving a
carboxylic acid and an alcohol; the products are the ester and water
Amines and amides
Amines are organic bases with general formula R3N.
Amides are composites of carbonyl and amine functionalities
25.7 Chirality in Organic Chemistry
Recall that molecules whose mirror images are nonsuperimposable are chiral.
Chemists use the labels R– and S– to distinguish between enantiomers.
Many pharmaceuticals are chiral; often only one enantiomer is clinically active.
A mixture of two enantiomers in the same quantity is called a racemic mixture.