Title: 3. Organic Compounds: Alkanes and Cycloalkanes
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22.12 Drawing Chemical Structures
- Representations of Structural Formulas
- Dot formulas are more cumbersome to draw than
dash formulas and condensed formulas - Lone-pair electrons are often (but not always)
drawn in, especially when they are crucial to
the chemistry being discussed
3- Dash formulas
- Each dash represents a pair of electrons
- This type of representation is meant to emphasize
connectivity and does not represent the
3-dimensional nature of the molecule
4- Condensed structures C-H and C-C and single
bonds aren't shown but understood - If C has 3 Hs bonded to it, write CH3
- If C has 2 Hs bonded to it, write CH2 and so
on. The compound called 2-methylbutane, for
example, is written as follows - Horizontal bonds between carbons aren't shown in
condensed structuresthe CH3, CH2, and CH units
are simply but vertical bonds are added for
clarity
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6- Condensed Structural Formulas
- In these representations, some or all of the dash
lines are omitted - In partially condensed structures all hydrogens
attached to an atom are simply written after it
but some or all of the other bonds are explicitly
shown - In fully condensed structure all bonds are
omitted and atoms attached to carbon are written
immediately after it - For emphasis, branching groups are often written
using vertical lines to connect them to the main
chain
7- Three-Dimensional Formulas
- The conventions for this are
- Bonds that lie in the plane of the paper are
indicated by a simple line - Bonds that come forward out of the plane of the
paper are indicated by a solid wedge - Bonds that go back out of the plane of the paper
are indicated by a dashed wedge
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9Classes of Compounds
- Classification based on functional group
- Three broad classes
- Hydrocarbons
- Compounds containing oxygen
- Compounds containing nitrogen
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10Hydrocarbons
- Alkane single bonds, sp3 carbons
- Cycloalkane carbons form a ring
- Alkene double bond, sp2 carbons
- Cycloalkene double bond in ring
- Alkyne triple bond, sp carbons
- Aromatic contains a benzene ring
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11Compounds Containing Oxygen
- Alcohol R-OH
- Ether R-O-R'
- Aldehyde RCHO
- Ketone RCOR'
12Cyclic ethers and ketones
THF (tetrahydrofuran)
cyclopentanone
13Carboxylic Acids and Their Derivatives
- Carboxylic Acid RCOOH
- Acid Chloride RCOCl
- Ester RCOOR'
- Amide RCONH2
14Cyclic esters
Cyclic ester
(cyclic ether and cyclic ketone)
15Compounds Containing Nitrogen
- Amines RNH2, RNHR', or R3N
- Amides RCONH2, RCONHR, RCONR2
- Nitrile RCN
16 Summary of Important Families of Organic
Compounds
17 183.2 Alkanes and Alkane Isomers
- Alkanes Compounds with C-C single bonds and C-H
bonds only (no functional groups) - The formula for an alkane with no rings in it
must be CnH2n2 where the number of Cs is n - Alkanes are saturated with hydrogen (no more can
be added - They are also called aliphatic compounds
19Alkane Isomers
- CH4 methane, C2H6 ethane, C3H8 propane
- The molecular formula of an alkane with more than
three carbons can give more than one structure - C4 (butane) butane and isobutane
- C5 (pentane) pentane, 2-methylbutane, and
2,2-dimethylpropane - Alkanes with Cs connected to no more than 2
other Cs are straight-chain or normal alkanes - Alkanes with one or more Cs connected to 3 or 4
Cs are branched-chain alkanes
20Constitutional Isomers
- Isomers that differ in how their atoms are
arranged in chains are called constitutional
isomers - They must have the same molecular formula to be
isomers
213.4 Naming Alkanes
- Compounds are given systematic names by a process
that uses - Prefix-Parent-Suffix
- Follows specific rules
- Named as longest possible chain
- Carbons in that chain are numbered in sequence
- substituents are numbered at their point of
attachment - Compound name is one word (German style)
- Complex substituents are named as compounds would
be
22Names of Small Hydrocarbons
23- Nomenclature of Branched-Chain Alkanes (IUPAC)
- Locate the longest continuous chain of carbons
this is the parent chain and determines the
parent name. - Number the longest chain beginning with the end
of the chain nearer the substituent - Designate the location of the substituent
- When two or more substituents are present, give
each substituent a number corresponding to its
location on the longest chain - Substituents are listed alphabetically
24- When two or more substituents are identical, use
the prefixes di-, tri-, tetra- etc. - Commas are used to separate numbers from each
other - The prefixes are used in alphabetical
prioritization - When two chains of equal length compete to be
parent, choose the chain with the greatest number
of substituents - When branching first occurs at an equal distance
from either end of the parent chain, choose the
name that gives the lower number at the first
point of difference
253.3 Alkyl Groups
- Alkyl group remove one H from an alkane (a part
of a structure) - Naming replace -ane ending of alkane with -yl
ending - ?CH3 is methyl (from methane)
- ?CH2CH3 is ethyl from ethane
- See Table 3.4 for a list
26Physical Properties
- Boiling points and melting points increase as
size of alkane increases - Forces between molecules (temporary dipoles,
dispersion) are weak
273.6 Cycloalkanes
- Cycloalkanes are alkanes that have carbon atoms
that form a ring (called alicyclic compounds) - Simple cycloalkanes rings of ?CH2? units, (CH2)n,
or CnH2n - Structure is shown as a regular polygon with the
number of vertices equal to the number of Cs (a
projection of the actual structure)
cyclobutane
cyclohexane
cyclopentane
cyclopropane
28Complex Cycloalkanes
- Naturally occurring materials contain cycloalkane
structures - Examples chrysanthemic acid (cyclopropane),
prostaglandins (cyclopentane), steroids
(cyclohexanes and cyclopentane)
296. Alkenes Structure and Reactivity
30Alkene - Hydrocarbon With Carbon-Carbon Double
Bond
- Also called an olefin but alkene is better
- Includes many naturally occurring materials
- Flavors, fragrances, vitamins
316.3 Naming of Alkenes
- Find longest continuous carbon chain containing
the double bond and name the compound using the
suffix ene - Number carbons in chain so that double bond
carbons have lowest possible numbers - Rings have cyclo prefix
32Names of Small Alkene
33Many Alkenes Are Known by Common Names
- Ethylene ethene
- Propylene propene
- Isobutylene 2-methylpropene
- Isoprene 2-methyl-1,3-butadiene
346.3 Degree of Unsaturation
- Relates molecular formula to possible structures
- Degree of unsaturation number of multiple bonds
or rings - Formula for saturated a acyclic compound is
CnH2n2 - Each ring or multiple bond replaces 2 H's
35Example C6H10
- Saturated is C6H14
- Therefore 4 H's are not present
- This has two degrees of unsaturation
- Two double bonds?
- or triple bond?
- or two rings
- or ring and double bond
36Degree of Unsaturation With Other Elements
- Organohalogens (X F, Cl, Br, I)
- Halogen replaces hydrogen
- C4H6Br2 and C4H8 have one degree of unsaturation
- Oxygen atoms - if connected by single bonds
- These don't affect the total count of H's
37Degree of Unsaturation and Variation
- Compounds with the same degree of unsaturation
can have many things in common and still be very
different
38If C-N Bonds Are Present
- Nitrogen has three bonds
- So if it connects where H was, it adds a
connection point - Subtract one H for equivalent degree of
unsaturation in hydrocarbon
39Summary - Degree of Unsaturation
- Count pairs of H's below CnH2n2
- Add number of halogens to number of H's (X
equivalent to H) - Don't count oxygens (oxygen links H)
- Subtract N's - they have two connections
406.4 Cis-Trans Isomerism in Alkenes
- The presence of a carbon-carbon double can create
two possible structures - cis isomer - two similar groups on same side of
the double bond - trans isomer similar groups on opposite sides
- Each carbon must have two different groups for
these isomers to occur
41Cis, Trans Isomers Require That End Groups Must
Differ in Pairs
- 180rotation superposes
- Bottom pair cannot be superposed without breaking
CC
X
426.6 Sequence Rules The E,Z Designation
- Neither compound is clearly cis or trans
- Substituents on C1 are different than those on C2
- We need to define similarity in a precise way
to distinguish the two stereoisomers - Cis, trans nomenclature only works for
disubstituted double bonds
43Develop a System for Comparison of Priority of
Substituents
- Assume a valuation system
- If Br has a higher value than Cl
- If CH3 is higher than H
- Then, in A, the higher value groups are on
opposite sides - In B, they are on the same side
- Requires a universally accepted valuation
44E,Z Stereochemical Nomenclature
- Priority rules of Cahn, Ingold, and Prelog
- The Cahn-Ingold-Prelog convention is used to
assign the groups of highest priority on each
carbon - Compare where higher priority group is with
respect to bond and designate as prefix - E -entgegen, opposite sides
- Z - zusammen, together on the same side
45Ranking Priorities Cahn-Ingold-Prelog Rules
- Must rank atoms that are connected at comparison
point - Higher atomic number gets higher priority
- Br gt Cl gt O gt N gt C gt H
In this case,The higher priority groups are
opposite (E )-2-bromo-2-chloro-propene
46Extended Comparison
- If atomic numbers are the same, compare at next
connection point at same distance - Compare until something has higher atomic number
- Do not combine always compare
47Dealing With Multiple Bonds
- Substituent is drawn with connections shown and
no double or triple bonds - Added atoms are valued with 0 ligands themselves