Chapter 26

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Published on 16 Oct 2011
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Chapter 26:
26-1 organic compounds and structures: overview
- Organic compounds contain: H and C atoms or something with the
combination with other atoms like: N, O, S
- Carbon is chosen for special study for its ability to form strong covalent bond
with each other, and their ability for from straight lines, branches and chains
- Hydrocarbons: simplest organic compound made with only H and C (ex. CH4
methane)
- Dashed wedge line notation:
Ordinary lines used to show bonds that lie in the plane of the paper
Solid wedge: bonds that stick out towards the viewer, in front of the
plane of the paper
Dashed lines: bonds away from the viewer, behind the plane of the
paper
Skeletal isomerism:
- Isomers: compounds that have the same molecular formula but different
structural formula
- When isomers differ from their structural skeletons ( one is branched, one is
straight) it is said to be skeletal isomerism
Nomenclature:
- Saturated hydrocarbons: C-C bonds are single bonds only AKA Alkanes
- When there is 2 isomers, for the straight liner structure use for ex. Butane
and for the branched structure use for ex. Isobutene
- Steps for naming organic compounds: PAGE 1077! Learn it.
Positional Isomerism:
- Positional isomers: Isomers that differ in the position of for ex. Br atom on a
C chain
Functional Groups:
- Distinctive groups of atoms are called: functional groups
- Physical & chemical properties of the molecule depend on the functional
group
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26-2 Alkanes:
- Alkanes only have single covalent bonds thus saturated
- Formula: CnH2n+2
- Each alkane differs by a CH2 group ( methylene)
- Homologous series: molecules in which they only differ by a constant unit for
ex. –CH2-
- Homologous series usually have similar chemical and physical properties
- Boiling points are related to molecular mass and shape
- Intermolecular attraction between straight line molecules are the strongest
and thus have the highest BP
- Compounds that are more compact (branched) have a lower BP
Conformation:
- Alkane molecules are able to rotate with respect to the stigma bond
connecting them
- Eclipsed formation: when a molecule rotates: for ex. CH2 is directly behind
another CH2 when the molecule is viewed head on ( also, distanced
between the H atoms of the adjacent C atom is at min)
- Staggerd conformation: when a molecule rotates: for ex. H atoms are located
at max distance apart ( more stable than eclipsed)
- At room temp, both conformations are possible
- At lower temp, usually only staggerd conformation
Ring Structures:
- Alkanes in chain structure have the formula: CnH2n+2 and are called: aliphatic
- Alkanes that exist in a ring are said to be alicyclic & have the formula : CnH2n
- The 4 acicyclic compounds are: equilateral triangle (cyclopropane), square
(cyclobutane), pentagon (cyclopentane), hexagon (cyclohexane)
- Cyclopropane is very unstable
- Cyclopentane molecule’s most stable arrangement in one of the C to buckle
out of the plane of the other 4
Preparation of alkanes:
- Chief source of alkanes is: petroleum
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- Unsaturated hydrocarbons ( with double or triple bonds) can be converted to
alkanes with the addition of an H atom to the multiple bond
Reaction of the Alkanes:
- Saturated hydrocarbons have little affinity for most chemical reactants
- Non-polar substances, insoluble in water, unreactive towards: acids, bases,
and oxidizing agents
- Most commonly encounterd reaction of alkanes is with O because alkane
bruns ( used as fuel)
Alkanes from Petroleum:
- Lower molecular mass alkanes ( methane & ethane) found in natural gas
- Propane and butane found dissolved in petroleumcan be extracted as gas
or solid from liquidized petroleum gas
- Higher alkanes are obtained by fractional distillation of petroleum
- Gasoline obtained by fractional distillation has a octane number of 50-55
which is not acceptable to be used in automobile because it needs fuels with
an octane number of 90ish
- Thermal cracking: large hydrocarbon being broken down into molecules in
the gasoline range
- Reforming or isomerisation converts straight chained hydrocarbons into
branched hydrocarbons having higher octane #s
- Alkylation: process of joining lower mass hydrocarbons into higher mass
hydrocarbons
26-3 alkenes and alkynes:
- Hydrocarbon that contain some double or triple bonds between its C atoms
are said to be unsaturated
- Alkenes or olefin: have one double bond with the general formula CnH2n
- Alkynes: have one triple bond with the general formula CnH2n-2
- PAGE 1085: to name alkenes and alkynes
- Alkenes are similar to alkanes in physical properties : at room temp those
containing 2-4 C atoms are gases, 5-18 are liquids, more than 18 are solids
- Alkynes have a higher BP than Alkanes & alkenes
Geometric isomerisation:
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