hydrocarbons i
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Hydrocarbons I:Alkanes
IntroductionAlkanes are • the simplest organic molecules, consisting of only
carbon and hydrogen and with only single bonds between carbon atoms.
• are used as the basis for naming the majority of organic compounds (their nomenclature).
• have the general formula CnH2n+2.• are the simplest and least reactive hydrocarbon
species containing only carbons and hydrogens. • principal constituent of gasoline and lubricating oils,
are extensively employed in organic chemistry• lack of unsaturation, contains no double or triple
bonds, which are highly reactive in organic chemistry.
Structure classification
Saturated hydrocarbons can be:• linear (general formula CnH 2n + 2)
wherein the carbon atoms are joined in a snake-like structure
• branched (general formula CnH2n + 2, n > 3) wherein the carbon backbone splits off in one or more directions
• cyclic (general formula CnH2n, n > 2) wherein the carbon backbone is linked so as to form a loop.
Linear: Methane• simplest alkane• Methane, (CH4, one carbon bonded to four hydrogens) is • the simplest organic molecule. It is a gas at standard
temperature and pressure (STP).• The true three-dimensional form of methane does not have
any 90 degree angles between bonded hydrogens. The bonds point to the four corners of a tetrahedron, forming cos-1(-1/3) ≈ 109.5 degree bond angles.
Linear: Ethane
• two carbons singly bonded to each other with six hydrogens is called ethane• second simplest hydrocarbon molecule. It can be thoughtof as two methane molecules attached to each other, but with two fewer hydrogen atoms.
Drawing alkanes• When writing out the alkane structures, you
can use different levels of shorthand depending on the needs at hand in hand. For example, pentane can be written out. Its formula is C5H12.
o CH3–CH2–CH2–CH2–CH3,o CH3(CH2)3CH3,
Line drawing shorthand• Although non-cyclic alkanes are called straight-chain
alkanes they are technically made of kinked chains. • reflected in the line-drawing method. Each ending point
and bend in the line represents one carbon atom and each short line represents one single carbon-carbon bond.
• Every carbon is assumed to be surrounded with a maximum number of hydrogen atoms unless shown otherwise.
• Propane butane pentane hexane heptane
Conformations• Structural formula and the bond angles are
not usually sufficient to completely describe the geometry of a molecule.
• There is a further degree of freedom for each carbon – carbon bond: the torsion angle between the atoms or groups bound to the atoms at each end of the bond.
• The spatial arrangement described by the torsion angles of the molecule is known as its conformation.
Conformations
Conformation: Newman Projections
• Newman projections are drawings used to represent different positions of parts of molecules relative to each other in space. Remember that single bonds can rotate in space if not impeded. Newman projections represent different positions of rotating molecule parts.
• Conformers interconvert readily, normally thousands of times a second as parts of molecules spin.
• In the following drawings, methyl groups are on the front and back ends of the molecule and a circle represents all that lies between.
Conformation: Newman Projections
Conformation: Newman Projections
Conformation: Steric Effects
• Steric effects have to do with size. Two bulky objects run into each other and invade each others space. If we replace one or more hydrogen atoms on the above Newman projections with a methyl or other group, the potential energy goes up especially for the eclipsed conformations
• Lets look at a Newman projection of butane as it rotates counterclockwise around its axes.
Anti Eclipsed Gauche Eclipsed
Conformation: Steric Effects
Alkanes with unbranched carbon chains are simply named by the number of carbons in the chain. The first four members of the series (in terms of number of carbon atoms) are named as follows:
CH4 = methane = one hydrogen-saturated carbonC2H6 = ethane = two hydrogen-saturated carbonsC3H8 = propane = three hydrogen-saturated carbonsC4H10 = butane = four hydrogen-saturated carbons
Alkanes with five or more carbon atoms are named by adding the suffix -ane to the appropriate numerical multiplier, except the terminal -a is removed from the basic numerical term. Hence, C5H12 is called pentane, C6H14 is called hexane, C7H16
is called heptane and so forth.
Number of hydrogens to carbons
• This equation describes the relationship between the number of hydrogen and carbon atoms in alkanes:
• H = 2C + 2 where "C" and "H" are used to represent the number of carbon and hydrogen atoms present in one molecule. If C = 2, then H = 6.
• Many textbooks put this in the following format:
• CnH2n+2 where "Cn" and "H2n+2" represent the number of carbon and hydrogen atoms present in one molecule. If Cn = 3, then H2n+2 = 2(3) + 2 = 8.
• Low melting point• Low boiling point
o almost a linear relationship with the size(molecular weight) of the moleculeo rises 20–30 °C for each carbon added to the chaino A straight-chain alkane will have a boiling point
higher than a branched-chain alkane due to thegreater surface area in contact between adjacent molecules
o cycloalkanes tend to have higher boiling points than their linear counterparts
• Are colourless and odourless• Are not very reactive when compared with
other chemical species.• Increase in the number of carbons =
increase in melting point and boiling point
Chemical properties
• All alkanes react with oxygen in a combustion reaction, although they become increasingly difficult to ignite as the number of carbon atoms increases.• In the absence of sufficient oxygen, carbon
monoxide or even soot can be formed
Chemical properties
Isomers of Alkane
• Alkanes with more than three carbon atomscan be arranged in various different ways,forming structural isomers.
• The simplest isomer of an alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called the n-isomer
(n for "normal", although it is not necessarily the most common).
• The number of possible isomers increasesrapidly with the number of carbon atoms.
Examples
C1: methane only C2: ethane only C3: propane only C4: 2 isomers: n-butane and isobutane C5: 3 isomers: pentane, isopentane, and neopentane
Examples
C6: 5 isomers: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane C12: 355 isomers C32: 27,711,253,769 isomers C60: 22,158,734,535,770,411,074,184 isomers, many of which are not stable.
Examples
Examples
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