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제 2 장 - 알칸과 고리형 알칸(Cycloalkane)


Functional Groups

  • a specific arrangement of atoms
  • define chemical families
  • determine chemical properties
  • basis of nomenclature
  • organization of textbooks

Hydrocarbons

  • contain only C and H
  • functional groups:
  • only C-H and C-C single bonds (alkanes)
  • C=C double bond (alkenes)
  • C=C triple bonds (alkynes)
  • rings (cycloalkanes)
  • aromatic rings (arenes)
  • form basic carbon skeleton

Heteroatoms

  • Halogens ( X = F, Cl, Br, I )
  • Oxygen ( C-O , C=O )
  • Nitrogen ( C-N , C=N , C=N )
  • Sulfur ( C-S , C=S )
  • lots of other possibilities
  • combinations of bonds
  • other elements

Groups with Single Bonds

  • R = general carbon group
  • halides ( R-X )
  • alcohols ( R-OH )
  • ethers ( R-O-R )
  • amines ( R-N )
  • thiols ( R-SH )
  • sulfides ( R-S-R )

Groups with Multiple Bonds

  • carbonyl families ( C=O )
  • imines ( C=N )
  • nitriles ( C=N )

Carbonyl Families

  • carbonyls:
    aldehydes ( R-CO-H )
    ketones ( R-CO-R )
  • carboxyls:
    carboxylic acids ( R-CO-OH )
    esters ( R-CO-OR )
    amides ( R-CO-NHR )

Alkane Family

  • methane CH4
  • ethane CH3CH3
  • propane CH3CH2CH3
  • butane CH3CH2CH2CH3
  • alkane CH3(CH2)nCH3
  • CnH2n+2 (homologous series)

Higher Alkanes

  • pentane C5H12
  • hexane C6H14
  • heptane C7H16
  • octane C8H18
  • nonane C9H20
  • decane C10H22
  • note the Greek prefixes

Alkane Isomers

  • carbon skeletons
  • straight-chain
  • branched-chain
  • cyclic chain
  • constitutional isomers
  • atoms connected in a different order
  • butane and isobutane
  • 3 pentane isomers

Butane isomers

n-butane

isobutane

Pentane Isomers

  • n-pentane
  • isopentane
  • neopentane

Alkyl Groups

n-propyl alcohol

isopropyl alcohol

(constitutional isomers)

Butyl Groups

  • n-butyl
  • sec-butyl
  • isobutyl
  • tert-butyl

Classification of C Atoms

  • 1° - primary - bonded to one other C
  • 2° - secondary - bonded to 2 other C's
  • 3° - tertiary - bonded to 3 other C's
  • 4° - quaternary - bonded to 4 other C's
  • class also applies to H atoms or functional groups attached to that C

Identifying Carbon Classes

  • identify the classes of each C, H and functional group in the molecule below

IUPAC Nomenclature

(prefixes)-(parent alkane)-(suffixes)
(substituents)-(longest chain)-(family)
Cl-CH2CH2-OH
2-chloro ethan ol

IUPAC Rules

  • parent = longest continuous carbon chain
  • number chain from end closest to a substituent (1st difference)
  • assign numbers to each subst.
  • in naming, arrange substituents alphabetically
  • for multiple substituents, use prefixes di-, tri-, tetra-, etc.
  • but they don't count when alphabetizing
  • separate names from numbers with hyphens, numbers from numbers with commas
  • otherwise all written as one word

IUPAC Examples

  • name the three isomers of pentane

pentane
2-methylbutane
2,2-dimethylpropane

IUPAC Examples

1-bromo-2,2,3-trimethylbutane

IUPAC Examples

3-ethyl-2,5,5,7-tetramethylnonane

Conformations

  • different 3D structures of the same molecule, differing only by rotations about single bonds

Ethane Conformations

C-H bonds on the two carbons may or may not align

eclipsed: C-H bonds are aligned

staggered: C-H bonds fit in between

Newman Projections

  • view down a C-C bond to visualize orientations

  • staggered conformation (more stable)

Potential Energy Diagrams

  • view energy changes as the C-C bond rotates

Writing Skeletal Structures

  • omit C-H bonds
  • assume C makes 4 bonds
  • omit C atoms
  • assume C at end of every bond
  • especially useful for cyclic structures
  • be able to put back all the details

Practice with Line Structures

methylcyclohexane

tryptophane

molecular formula is: C11H12N2O2

Cycloalkanes

  • cyclo-prefix
  • parent alkane is the ring size
  • number so as to get lowest sum
  • 1-methyl-3-propylcyclopentane

Cis-Trans Isomers

  • stereoisomers: differ in 3D orientation, even though all atoms have the same connections
  • isomers of dimethylcyclopropane:

Small Rings

  • typically unstable due to:
  • angle strain
    bent bonds
  • eclipsing strain
  • cyclopropane - planar
    bond angles 60°
  • cyclobutane - puckered

Common Rings

  • cyclobutane - puckered
  • cyclopentane - envelope
  • cyclohexane - chair

Cyclohexane

  • all bonds staggered
  • all bond angles 109°
  • practice drawing a good chair
    opposite C-C bonds parallel
    C-H bonds are straight up or down or parallel to a C-C bond

Substituted Cyclohexanes

  • axial and equatorial positions
  • the chair can flex
    axial equilibrates with equatorial

  • equatorial is the preferred position
    1,3-diaxial interferences (steric)
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