Introduction to Alkanes

• Alkanes are discussed

Organic vs. Inorganic Compounds

• Early definitions of organic compounds included that they were obtained from living organisms.
• Early definitions of inorganic compounds included that they were obtained from mineral constituents of the earth.
• Current definitions of organic compounds include carbon-containing compounds and their derivatives.
• Current definitions of inorganic compounds include non-carbon containing compounds.
• Organic chemistry is the study of hydrocarbons and their derivatives.
• There are approximately 7 million known organic compounds.
• Organic compounds are no longer derived from living organisms.
• Organic compounds are synthesized in the lab.
• Inorganic chemistry is the study of all other compounds except hydrocarbons and their derivatives.
• There are approximately 1.5 million known inorganic compounds.
• The ratio of organic to inorganic compounds is almost 5:1.

Carbon's Special Properties

• Carbon can make a large number of different compounds.
• Carbon is a Group IVA element.
• Carbon needs 4 electrons for a full valence shell.
• Carbon must have 4 bonds.
• Carbon can make 4 bonds with predictable geometries
• Combinations include, 4 single bonds (Tetrahedral geometry), 2 single + 1 double bonds (Trigonal planar geometry), 2 double bonds (Linear geometry), and 1 single + 1 triple (Linear geometry).
• Carbon can bond with other carbons to form long chains and ring structures

Hydrocarbons

• Hydrocarbons contain carbon and hydrogen only.
• There are two classes: saturated and unsaturated.
• Saturated hydrocarbons contain only single carbon-carbon bonds.
• The general formula for acyclic alkanes is CnH(2n+2).
• Unsaturated hydrocarbons contain double and/or triple carbon-carbon bonds.
• For every double bond, lose 2 hydrogens.
• For every triple bond, lose 4 hydrogens.
• Acyclic hydrocarbons have all carbons linked in a single chain, can be linear or branched.
• Cyclic hydrocarbons have carbon atoms bonded, forming a ring structure.
• For cyclic structures, the general formula loses 2 hydrogens.

Drawing Structures

• Structural formulas give information on how the atoms are connected in the molecule.
• Expanded form shows all elements and all bonds.
• Condensed form shows atom groupings, which are related to specific connection patterns.
• Skeletal structure or carbon chain shows only carbon atoms with no other elements.
• It is understood that carbon will have a total of 4 bonds
• Missing bonds are filled with hydrogens to make 4 bonds to each carbon.
• Line-angle structure or stick structure uses lines as the bonds between carbon atoms and carbon atoms are the connection points.
• The number of hydrogen atoms will give each carbon 4 bonds total.
• All other elements are shown in the structure.
• Expanded form shows the connectivity for all atoms in the molecule and the bond types connecting the elements.

Constitutional Isomers

• Constitutional isomers are different compounds with the same chemical formula, but the atoms are connected differently.
• Constitutional isomers have the same chemical formula.
• There is more than one structural formula.
• Constitutional isomerism begins with C4H10 structures and increases from there.
• The number of isomers grows exponentially as the number of carbon atoms in the molecule increases.

Conformations

• Conformations are the specific 3-D arrangement of atoms in a molecule resulting from free rotation about a carbon-carbon single bond.
• Free rotation gives rise to multiple shapes of the carbon chain.
• Different conformations represent the same molecule.
• Free rotation does not occur with double or triple bonds.

Naming Alkanes

• Molecules might have multiple names, but they are named according to IUPAC.
• There are common names to become familiar with.
• The IUPAC naming system is a method for naming all organic compounds.
• Knowledge of this system builds as the semester progresses.
• Suffix for Alkanes is "ane" as they are saturated hydrocarbons
• To name straight chain alkanes, know how many carbons are linked in a single chain.

IUPAC Naming Rules for branched alkanes

• Find the longest carbon chain
• Identify branching points
  • Identify the substituents (branches)
  • Find the number of carbons in each branch
• Give the substituent the lowest number location or number combination
  • If you cannot distinguish, then prioritize alphabetically
• List alphabetically in name

IUPAC naming format

• All Substituents - parent chain - family name ending
• Substituent location and name
  • Substituent location and name separated by a dash
  • If more than one substituent
    • List alphabetically
    • If substituents are the same
      • Use prefixes di, tri tetra... to indicate how many substituents are the same
      • Prefixes not used when alphabetizing
      • List each location number and separate location numbers with commas
• Parent chain – how many carbons
• Family name ending – “ane” for alkanes

Carbon Classifications

• Primary (1°) - bonded to one other carbon; other bonds are to hydrogen, usually found at chain end, CH3 group
• Secondary (2°) - bonded to two other carbons; other bonds are to hydrogen, CH2 group
• Tertiary (3°) - bonded to three other carbons; other bond is to hydrogen, CH group
• Quaternary (4°) - bonded to four other carbons, no bonds to hydrogen

Cycloalkanes

• Cycloalkanes = cyclic compounds
• Cycloalkanes - a saturated hydrocarbon with a ring structure (cyclic arrangement)
• Minimum number of carbons = 3, range is between 3-30
• General formula for cycloalkanes: CnH2n
• General formula for acyclic alkanes: CnH2n+2
• Tetrahedral geometry = 109.5
• IUPAC Naming guidelines are the same as naming straight and branched chain alkanes
  • The parent chain uses the number of carbons in the ring structure
  • Substituent location number depends on the number of substituents
    • 1 substituent - no # needed - If no C#1 in a circle, it's understood that the substituent is C#1 - 2 substituents - alphabetical priority determines C#1
      • Numbering of Carbons follows in the direction to give the lowest location numbers to the other substituent
    • 3 or more substituents - lowest number combination or if needed use alphabetical priority

Cycloalkanes Isomers have Two Types

• Constitutional isomers - same chemical formula but connectivity of atoms is different
• Cis/Trans isomers - the same chemical formula with same connectivity, but different 3-D arrangement in space
  • Because the carbon atoms are held in a fixed location
• The carbon atoms or groups of atoms attached to those in the ring will have a set configuration based on their attachment - Creates a top and bottom in the structure These are called stereoisomers
• Cis - substituents on the same side of the ring.
• Trans - substituents on opposite sides of the ring Occurs in rings of all sizes Requires 2 substituents on 2 different carbons Substituents do not need to be the same Biochemically important This nomenclature is limited to ring structures that contain one hydrogen and one substituent on the substituted carbon atoms
  • More complicated systems use E,Z nomenclature system

Properties of Alkanes

• Insoluble in water because alkanes are nonpolar
• Hydrocarbons are nonpolar
• Density is less than water -Range from 0.6 – 0.8 g/mL Float on top of water=
• Boiling points
  • Lowest boiling points an melting points
  • Phases at room temps-
  • C1 - C4 are gases -C5 – C17 liquids -C17 solids
• Molar mass increases means the boiling point increases.
• Branched alkanes, have lower boiling point because decreased surface area contact
• Cycloalkanes, have higher boiling points compared to that of straight alkanes because better packing

Reactions of Alkanes, involves 2 types

• Combustion w/ O2 and Halogenation w/ Halogens are major reactions
• Lower reactivity than other classes of organic compounds
  • Combustion reaction produces carbon dioxide and water.
  • General reaction: 2 C2H6 + 7 O2 -> 4 CO2 + 6 H2O -Product of incomplete combustion is Carbon Monoxide
• Halogenation Reaction, incorporates a halogen atom( F, Cl, Br or I ) into a molecule Substitution reaction – removing an atom or group of atoms to replace it with other For alkanes, Hydrogen atoms have being replaced w/ halogen in its reaction The general reaction is: R-CH3 + X₂ -> R-CH₂X + HX (Heat/light & catalyst required).
  • R = rest of molecule & X = halogen atom
    • Which can be H, methyl group, ethyl group -H alogenation reaction forms a mixture of products. Not limited to remove specificity of hydrogen atoms,Any hydrogens removed. . , Major product/s from (major prod based on stoichiometric, so still mix)

Naming Halogenated Alkanes:

• Rules are the same as for other alkanes
• Halogen is a substituent
• Prioritized like other alkyl substituents
  • Location number or alphabetical
  • Substituent names for halogens • fluoro = F • chloro = Cl • bromo = Br • iodo = I

Properties of Halogenated Alkanes

• Higher Boiling Points from Alkanes
  • CX bond is polar, making Dipole - induced dipole stronger than the London Dispersion Forces that Hydrocarbons exhibit
• Higher Densities from Alkanes
  • Halogens have a higher atomic mass than carbons
• Haloalkanes w/ >2 Chlorines, Bromines, or Iodine have a greater density than water