CHM 51Chapter 20 – Organic Chemistry
20.1 -20.3 Organic compoundExamples
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•is a compound made from carbon atoms
•has one or more C atoms.
•has many H atoms.
•may also contain O, S, N, and halogens
•foods, flavors and fragment
•Medicines, toiletries and cosmetics
•Plastics, films, fibers and resins
•All other living organisms
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Where do we obtain organic molecules?
Two principal ways:
◦isolation from nature
◦synthesis in the laboratory
Typical organic compounds
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•have covalent bonds.
•have low melting points.
•have low boiling points.
•are flammable.
•are soluble in nonpolar solvents.
•are not soluble in water.
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Differences Between Organic and Inorganic Compounds
- Organic compounds are easily decomposed into simpler substances by heating, but inorganic substances are not.
•Inorganic compounds were readily synthesized in the lab, but synthesis of organic compounds in the lab is hard.
What’s So Special About Carbon?
•Carbon atoms can do some unique things that other atoms cannot.
•Carbon can bond to as many as four
other atoms.
•Bonds to carbon are very strong and nonreactive.
•Carbon atoms can attach together in long chains.
•Carbon atoms can attach together to form rings.
•Carbon atoms can form single, double, or triple bonds.
Summary
•Hydrocarbons contain only C and H.
–Aliphatic or aromatic
•Insoluble in water
–No polar bonds to attract water molecules
•Aliphatic hydrocarbons
– Saturated or unsaturated aliphatics
•Saturated = alkanes; unsaturated = alkenes or alkynes
–May be chains or rings
–Chains may be straight or branched.
Aromatic hydrocarbons
Method of Drawing Structures
Lewis Structure
Condensed Structure
Bond-line Drawings (or stick figures) are short hand way to draw carbon structures
- end points and intersections represent C atoms
- omit H’s attached to C’s
More examples
Constitutional isomers: Compounds that have the same molecular formula but different structural formulas (a different connectivity of their atoms).
•For the molecular formulas CH4, C2H6, and C3H8, only one structural formula is possible. There are no constitutional isomers for these molecular formulas.
E.g. How many alkane structures can you draw from C4H10? (*Hint: always start with a straight chain carbon-carbon backbone)
E.g. How many isomers can you draw from C4H11Cl?
Do cycloalkanes have isomers? YES
E.g. There are two possible ways to make ring using four carbon atoms of C4H8.
E.g How many cyclic constitutional isomer can you draw from C5H10?
20.4 – Alkane: Saturated Hydrocarbon
Alkanes (RH) / Alkyl Substituent (R-) / Other Substituent (R-)(“parent” carbon chains)
CH4 methane
CH3CH3 ethane
CH3CH2CH3 propane
CH3(CH2)2CH3 butane
C5 pentaneC8 octane
C6 hexaneC9 nonane
C7 heptaneC10 decane
/ (groups attached to parent)
CH3 meth (Me)
CH2CH3 ethyl (Et)
CH2CH2CH3 propyl (Pr)
/ (groups attached to parent)
F fluoro
Cl chloro
Br bromo
I iodo
IUPAC Rules for naming alkanes
- Find the longest carbon chain (if there is a tie, choose chain with the most substituent). Name parent
- Number the carbon chain, starting from the end closest to the first substituent
- Name and number the subtituents (use di, tri, tetra etc.., prefixes for groups that appear more than once).
- Alphabetize and list substituents before the parent name. Ignore all prefixes other than iso.
E.g.
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20.5Physical Properties of
Alkenes
•Pi bond electrons not held as tight as sigma; therefore, alkenes are more polarizable than alkanes.
•Cis generally more polar than trans
•Trans lower boiling point
•More carbon groups attached to the double bond = higher boiling point.
–For equal numbers of C
•Densities similar to alkanes
•Trans higher melting point than cis
–Molecules are more symmetrical and pack better.
Alkynes:
•Also known as acetylenes
•Aliphatic, unsaturated
•CºC triple bond
•Formula for one triple bond = CnH2n − 2.
–Subtract 4 H from alkane for each triple bond.
•Linear shape
•Internal alkynes have both triple bond carbons attached to C.
•Terminal alkynes have one carbon attached to H.
•Higher boiling points than similar sized alkenes
•Similar size = same number of carbons
•More pi bond = more polarization = higher boiling point
•Slightly higher densities than similar alkenes
•There are no alkyne cis or trans isomers.
•Internal alkynes have higher boiling points than terminal alkynes.
•With the same number of C
Naming Alkenes and Alkynes
To name an alkene;
- The parent name is that of the longest chain that contains the C=C.
- Number the chain from the end that gives the lower numbers to the carbons of the C=C.
- Locate the C=C by the number of its first carbon.
- Use the ending -eneto show the presence of the C=C
- Branched-chain alkenes are named in a manner similar to alkanes in which substituted groups are located and named
- Don’t forget to include the configuration where stereochemistry is appropriate (cis, trans, E, Z)
- A compound with two double bonds is called –diene (or triene, tetraen, etc..)
- If the structure contains a triple bond, it is named as an “#-alkyne.”
Geometric Isomerism
•Because the rotation around a double bond is highly restricted, you will have different molecules if groups have different spatial orientation about the double bond.
–Stereoisomers
•This is often called cis–trans isomerism.
•When groups on the doubly bonded carbons are cis, they are on the same side of the double bond.
•When groups on the doubly bonded carbons are trans, they are on opposite sides.
Misconception Alert!
cis Z and trans E
In general terms there is NO specific relationship between cis and trans / E and Z as they are based on fundamentally different naming rules.
The cis- / trans- style is based on the longest chain whereas the E/Z style is based on a set of priority rules. The E- and Z- style is more reliable and particularly suited to tri- or tetra-substituted alkenes, and especially when the substituents are not alkyl groups
*Greater atomic number => higher priority If two groups are the same consider the next atom and so on …
20.6 - Reactions of Hydrocarbons
•All hydrocarbons undergo combustion.
•Combustion is always exothermic.
–About 90% of U.S. energy generated by combustion
•Burning hydrocarbons releases heat and light energy.
–Combustion
•Alkane + oxygen carbon dioxide + water
•Larger alkane, more heat released
CH3CH2CH3(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)
CH2═CHCH2CH3(g) + 6 O2(g) → 4 CO2(g) + 4 H2O(g)
CH≡CCH3(g) + 4 O2(g) → 3 CO2(g) + 2 H2O(g)
Alkanes Reactions:
•Substitution
–Replace H with a halogen atom.
–Initiated by addition of energy in the form of heat or ultraviolet light
•To start breaking bonds
–Generally get multiple products with multiple substitutions
–Methane + chlorine chloromethane + HCl
E.g Predict all the possible products
CH3CH2CH3 + Br2 + light
Addition of Alkenes and Alkynes:
•Adding a molecule across the multiple bond
•Hydrogenation = adding H2
–Converts unsaturated molecule to saturated
–Alkene or alkyne + H2 → alkane
–Generally requires a catalyst
•Halogenation = adding X2
•Hydrohalogenation = adding HX
–HX is polar.
–When adding a polar reagent to a double or triple bond, the positive part attaches to the carbon with the most H’s.
•Addition of HX, H2, X2
X = halogen
E.g Predict the major product(s)
a.
b.
20.8 Functional Groups
•Other organic compounds are hydrocarbons in which functional groups have been substituted for hydrogens.
•A functional group is a group of atoms that shows a characteristic influence on the properties of the molecule.
–Generally, the reactions that a compound will perform are determined by what functional groups it has.
–Because the kind of hydrocarbon chain is irrelevant to the reactions, it may be indicated by the general symbol.
R—OH R—O—RR—SH
Alcoholetherthiolnitrile amine
Alcohols
Aldehydes and Ketones
Carboxylic acid
Ester
Ether
•Diethyl ether is the most common ether.
•It is useful as a laboratory solvent and can dissolve many organic compounds.
•It has a low boiling point.
Amine:
•N containing organic molecules
•Very bad smelling
•Form when proteins decompose
•Organic bases
•Name alkyl groups attached to the N, then add -amine to the end.
•Many amines are biologically active.
•Dopamine – a neurotransmitter
•Epinephrine – an adrenal hormone
•Pyridoxine – vitamin B6
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