Isomers and properties of organic compounds Chemistry III IB

Isomerism Isomers are chemical compounds that have the same chemical formula, but a different arrangement of the atoms in the formula that give it a different set of properties. Therefore two isomers will have two different names. They are two different compounds that share the same formula.

Structural Isomers Have a different arrangement ( bonding order) for the atoms of this chemical formula. For example C4H10 has isomers that differ in alkyl branching. Draw butane and 2-methyl propane. Confirm that they have same chemical formula, but different names therefore they are isomers.

Structural isomers of alkanes Pentane or n-pentane C5H12 There are 3 strutural isomers of this formula. Above is one. Find the other 2, draw and name.

Isomers of C6H14 Hexane (of course) Draw structural formulas and name the rest. 2-methyl pentane 3-methyl pentane 2,3-dimethyl butane 2,2-dimethyl butane

Structural isomers (position of functional group) 2-pentanone 3-pentanone Would 3-methyl-2-butanone be an structural isomer of these two other compounds or a position isomer? Why?

Other position isomers All functional groups except carboxylic and aldehyde can have structural position isomerism. Draw and name a 6 carbon ester. Draw and name two position isomers of your first ester. Do the same with a 6 C alkanol

Structural isomers (functional groups) C2H6O Has two isomers that differ by the functional group. Ethanol and methoxymethane are the two function group isomers What functional group goes with carboxylic acid? What functional group goes with aldehyde?

Functional group isomers C3H6O Draw and name two functional group isomers for this formula. Propanone Propanal

Draw and name all isomers of C3H6O2 (no double functional groups) Propanoic acid Methyl ethanoate

Think of all types of structural isomerism All the Isomers for C4H10O Think of all types of structural isomerism

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Geometric Isomerism Two different compounds that have the same chemical formula and bonding but differ in in only the 3-d arrangement of atoms in an area “locked into place by a double bond or ring structure. The pi bond locks this geometry into this trigonal planor shape around both carbons of the double bond No Free Rotation

Cis and trans prefixes Cis = Same When there are like species on each side of the double bond use the prefix cis and then the normal name. Trans = Across When there are unlike species on each side of the double bond use trans as a prefix and use the normal name.

Use of the cis and trans prefixes Cl and H on same side therefore Cis is used. Cis-1,2-dichloroethene Trans-1,2-dichloroethene

Names and formulas for geometric isomers Draw and name the two geometric isomers of 1,2 dinitroethene. Trans-1,2-dinitroethene Cis-1,2dinitroethene Is there a structural isomer of these two compound?

Geometric Isomers continued Yes, 1,1-dinitroethene is not a structural isomer of the cis and trans geometric isomers. But is not a geometric isomer. Draw all the isomers of C4H8 This arrangement does not have any geometric isomers. 1-butene or buta-1-ene

2-butene exists as two geometric isomers. More Isomers of C4H8 2-butene exists as two geometric isomers. Trans-2-butene or Trans-buta-2-ene M Cis-2-butene or Cis-buta-2-ene

One more C4H8 isomer Cyclobutane This compound has no geometric isomers. There are 4 isomers of C4H8

Isomers within ring compounds both geometric and structural. Since a ring formation does not allow for free rotation of the sigma bond, cis and trans formations are a possibility. Trans-1,2-dichloro cyclopentane Cis-1,2-diclorocyclopentane 2 Cl on the same side of the ring 2 Cl on oposite sides of ring

Optical isomerism Differ only in the arrangement of 4 different chemical species around a central carbon. A carbon with four different groups on it is called a chiral carbon. When a chiral carbon is present in a compound the compound will exist as two optical isomers.

Enantomiers of 1-bromo-1-chloro ethane The names of these two will differ by a prefix L or D at the start of the name. Use a tetrahedral arrangement around the chiral carbon. Enantomiers are non superimposable mirror images.

Drawing enantomers * Which carbon is the chiral? Star that C. One of these is L-3-methyl hexane and the other is D. Which is which is beyond the scope.

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Physical properties of organic compounds. Physical properties are measurable or observable aspects of a compound that tend to differ between compounds. Examples are boiling point and solubility in water. Most physical properties are a function of the strength of the inter particle forces between the molecules of the compound.

Optical Isomers differ in only one physical property Two enantiomers of an optically active compound will differ only in the direction that they rotate plane polorized light. Rotation to Right PPL filter Pure sample of R-… Non polarized photons oscillate on all planes

The other enantiomer The L-… enantiomer would rotate PPL to the left the an equal number of degree. The only physical difference is the direction of rotation of PPL. Most optically active compounds are found in nature as a 50 –50 mix of the L and D enantiomers. This is called a racemic mixture and does not rotate PPL.

Enantiomers continued There is only one chemical property difference. It involves only when the other substance is also an optical isomer. For example the L-… might only react with the D-*** and not react with the L-*** This happens often in biochemistry.

Boiling points of organic compounds Boiling point of a compound will increase as the intermolecular force (between molecules) increases Types of intermolecular forces a. Vanderwalls force - comes from an induced dipole interaction. (non-permanent dipole – dipole) Present in all substances but only important if a non-polar substance. ( only force present)

B.P. continued / Vanderwalls Vanderwalls force increases with increasing molecular weight (size). Bigger molecule will have a larger induced dipole. Larger partial charge larger force of attraction. Coulombs law - Force = K q1q2/r2 as charge gets larger or distance between charges gets smaller force will increase.

Examples Butane has a higher b.p. than propane. Why? Non-polar isomers have same MW. However more branches make molecules further apart and weaker force. Lower B.P.

Dipole – Dipole b. Dipole – Dipole forces come from a permanent partial charge that develops due to E.N. difference and a molecular shape that allows it to develop. Partial negative Cl in one molecule attracts partial positive C in the other molecule.

Functional groups with dipole’s If MW is similar, then the rank goes ether, ester, aldehyde and ketone is strongest of this group.

Ranking B.P. or M.P. Consider the following Butene, propanone and methylmethanoate, rank them from lowest to highest b.p. and explain. Since all three have similar MW they will have similar strength of Vanderwalls force. Butene is non-polar and has only vanderwalls force and will have the lowest B.P. Propanone and the ester both have dipole, however propanone has the higher partial charge and therfore stronger dipole and highest B.P. of these compounds.

H-bonding the strongest dipole c. H-bonding When a molecule has a H bonded to a N, O or F it will have the strongest interparticle force called hydrogen bonding. Strong due to two factors. 1. N,O and F have high EN and set up a large partial charge with H and 2. The small size of H allows the charges to be close together.

Functional groups that H-bond. Three groups fit the H-bonding model All are stronger than just dipole interactions Carboxylic is strongest partial charge due to neighboring oxygen’s E.N. Therfore strongest H-bond. Amine is the weakest. N has lowest E.N. therfore weakest H-bond.

Ranking Rank the following compounds from low to high Boiling point and state the main interparticle force and draw a structural formula including partial charges Ethanoic acid, butane, 2-methylpropane, propan-1-ol, Propylamine, methylmethanoate.

Solubility of Organic Compounds In Water the more polar areas in a compound the more soluble in water. WHY? Small alkanols are very soluble. As the hydrocarbon chain increases solubility decreases.

Hydrophobic vs Hydrophilic Equals not very soluble. Large hydrophobic Small hydrophilic If this compound had multiple hydroxyl groups then its solubilty would increase. H-bonding is more soluble than just polar.

Ranking Solubility Consider butan-1-ol, butanal and butanoic acid. Rank and explain the relative solubility in water. Give a 4 carbon compound that would be less soluble than the 3 given above. Defend your choice.

ACIDIC NATURE - CHEMICAL PROPERTY Ka and pKa ACIDIC NATURE - CHEMICAL PROPERTY HA(aq) H+(aq) + A-(aq) Ka = 1.2 x 10-6 pKa = - log(1.2 x 10-6) = 5.92 As Ka increases pKa decreases and strength of acid increases Which functional group is the most acidic?

Carboxylic

Acidic Organic compounds Acidic = Proton donor ( Bronsted) Any partial positive H in a molecule has some acidic nature. (acidic H). The amount of this donation to water is a measure of the strength of the acid. All organic compounds that have acidic nature are weak. What does that mean?