11. Alkanes Flashcards
Alkane
-saturated hydrocarbon whose carbon atoms are arranged in a chain
-maximum number of hydrogens on each carbon
-C atoms are tetrahedral (109.5°)
-ALIPHATIC HYDROCARBONS: physical properties of the higher members of this class resemble those of the long carbon-chain molecules in animal fats and plant oils
Hydrocarbon
-compound that contains only carbon and hydrogen atoms
•alkanes (single bond C-C)
•alkenes (double bond)
•alkynes (triple bond)
•arenes (one or more benzene-like rings)
Saturated hydrocarbon
-hydrocarbon that contains only C-C single bonds
Unsaturated hydrocarbon
-hydrocarbon that contains one or more C-C double/ triple bonds or benzene ring
Line-angle formula
-Abbreviated way to draw structural formulas in which each vertex and line terminus represents a carbon atom and each line represents a bond
Constitutional isomers
aka
Structural isomers
-compounds with the same molecular formula but a different connectivity of their atoms
-i.e. butane and 2-methylpropane
International Union of Pure and Applied Chemistry (IUPAC) system
NAMING ALKANE
•for unbranched alkane
1. Prefix: number of carbon atoms in chain
2. Suffix -ane
•for branched alkane
1. Take the longest chain as parent chain and its name becomes the root name
- Name and number each substituent on parent chain. Use hypen to connect number to name
2-Methylpropane - If there is one substituent, number the parent chain from the end that gives the substituent the lower number
2-Methylpentane NOT 4-Methylpentane - If same substituent occurs more than once, number the parent chain from the end that gives the lower number to the substituent encountered first. Use appropriate prefix (i.e. di-, tri-, tetra-). Use comma to separate number
2,4-Dimethylhexane - If two or more different substituent, list in alphabetical order and number the chain that gives lower number to first substituent encountered.
If equivalent positions, give the substituent of lower alphabetical the lower number
3-Ethyl-5-methylheptane NOT 3-Methyl-5-ethylheptane - Do not include prefixes or hypenated prefixes (sec-, tert-) in alphabetizing
4-Ethyl-2,2-dimethylhexane
Prefix
Meth
Eth
Prop
But
Pent
Hex
Hept
Oct
Non
Dec
Undec
Dodec
Tridec
Tetradec
Pentadec
Hexadec
Heptadec
Octadec
Nonadec
Eicos
Alkyl group
-group derived by removing a hydrogen from an alkane; symbol
—R
-named by dropping -ane from the name of parent alkane and add -yl
-prefix sec- (abbv secondary): carbon bonded to two other carbons
-prefix tert- (abbv tertiary): carbon bonded to three other carbons
Common names
-total number of carbon atoms , regardless of arrangement, determines the name
Cycloalkane
-saturated hydrocarbon that contains carbon atoms bonded to form a ring
•Naming:
1. Prefix the corresponding open-chain alkane with cyclo- and name substituents
2. No need to number if one substituent. If more than one, number the ring beginning the substituent of lower alphabetical order
Conformation
-any three-dimensional arrangement of atoms on a molecule that results from rotation about a single bond
Conformation
ALKANE
-majority of molecules in any alkane sample will be in the LEAST CROWDED CONFORMATION
-fewest will be in MOST CROWDED CONFORMATION
Conformation
CYCLOALKANE (CYCLOPENTANE)
-ENVELOPE CONFORMATION (most stable): 4 carbons are in a plane and the 5th carbon is bent out of the place (like an envelope flap)
-bond angle 109.5°
Conformation
CYCLOALKANE (CYCLOHEXANE)
-CHAIR CONFORMATION (most stable): 12 C-H bonds are arranged in two different orientations:
•Equatorial position- extends from the ring roughly perpendicular to imaginary axis of the ring
-if axial bond upward, equatorial slightly down vice versa
-alternating per carbon
•Axial position- extends from the ring parallel to the imaginary axis of the ring
-109.5°
-cyclohexane more stable if maximum number of substituent groups are EQUATORIAL
Cis-trans isomers
aka
Geometric isomers
-isomers that have the same connectivity of their atoms but a different arrangement of their atoms in space due to the presence of either a ring or C-C double bond
-uses solid wedges (project toward you); broken wedges (project away from you)
Cis-
-Latin: on the same side
-substituents are on the same side of the ring
Trans-
-Latin: across
-substituents are on opposite side of the ring
Stereocenter
-tetrahedral atom, most commonly carbon, at which exchange of two groups produces stereisomer
Stereoisomers
-isomers that have same connectivity of their atoms but different orientation of their atoms in space
-(i.e.)cis-trans isomers and enantiomers
Configuration
-refers to arrangement of atoms about a stereocenter, that is, relative arrangement of parts of a molecule in space
PHYSICAL PROPERTIES OF ALKANES
1. Melting and boiling points
-boiling point of alkanes are lower than almost any other type of compound of same molecular weight
-⬆️ Molecular weight ⬆️ B.pt. M.pt.
-Gases at room temperature: 1-4 carbons
-Colorless liquid: 5-17 carbons
-White, waxy solids: 18+ carbons
-constitutional isomers have different physical and chemical properties
-more branching, more compact, surface area decreases, lower boiling point
PHYSICAL PROPERTIES OF ALKANES
2. Solubility
-alkanes non polar, not soluble in water
-cannot form hydrogen bond with water
PHYSICAL PROPERTIES OF ALKANES
3. Density
-all liquid and solid alkanes are LESS DENSE THAN WATER
-floats in water since insoluble
Reaction with Oxygen: COMBUSTION
-basis for their use as energy sources for heat and power
-HEAT OF COMBUSTION: heat liberated when alkane is oxidized to CO2 and H2O
Methane
-major component of natural gas
Propane
-major component of LPG or bottled gas
Reactions with Halogens: HALOGENATION
-uses heat/ light
-substitution reaction
-IUPAC name for haloalkanes by naming halogen atom as substituent (bromo-, chloro-) and alphabetizing it along with other substituents
-common name: name of alkyl group followed by name of halogen
Dichloromethane
Methylene chloride
IMPORTANT HALOALKANES
1. Chlorofluorocarbons (CFCs)
-trade name: FREONS
-nontoxic, nonflammable, odorless, non-corrosive
-heat transfer agents in refrigeration system, industrial cleaning solvents, propellants in aerosol sprays
IMPORTANT HALOALKANES
2. Solvents
-low molecular weight haloalkanes: excellent solvents for organic reactions
-use as cleaners, degreasers
-carbon tetrachloride
Alkanes sources
- Natural gas
methane, ethane, and other low boiling alkanes - Petroleum-thick, viscous, liquid mixture of thousands of compounds (mostly hydrocarbons) formed from decomposition of marine plants and animals
-fracrional distillation
-provides fuel, greases and lubricant
•Both provide 90% of organic raw materials for synthesis and manufacture of synthetic fibers, plastics, detergents, drugs, dyes, etc
Octane rating
-Gasoline
-quality as fuel for internal combustion engines depends on how much it makes an engine ‘knock’ (air-fuel mixture explodes prematurely and independently of ignition by spark plug)
-2,2,4-Trimethylpentane (isooctane): very good antiiknock and heptane (poor antiknock)
•octane rating: percentage of isooctane in mixture of isooctane and heptane
