The properties and characteristics of organic compoundsUnit 1 Organic Chemistry

Intermolecular forcesThe physical and chemical properties are determined by the bonds and forces between atoms and molecules.Intramolecular forces occur between atoms in molecules and determine a substances chemical properties.Intermolecular forces occur between molecules and affect a substances physical properties.

Intermolecular forcesThe physical properties of organic compounds are affected by the intermolecular forces of attraction and repulsion between moleculesLarger forces of intermolecular attraction result in high boiling & melting points.Weak intermolecular forces result in low boiling & melting points producing gas states.

Intermolecular forcesThe foundation for these intermolecular forces lies in the types of bonds that form between atoms in molecules. The bonding continuum ranges from non-polar to ionic, however organic molecules are predominantly covalent molecules and lie in the polar to non-polar range.Polar and non-polar regions affect solubility

Intermolecular forcesThe polarity of a bond is determined by the electronegativity difference.Electronegativity Difference (DE)

Intermolecular forcesThe intermolecular forces are attractive and repulsive forces between molecules determined by factors such as:The bond types within the molecules.Polar vs. Non-polarThe size of the molecules.The shape of the molecules.

Intermolecular forcesJohannes van der Waals (1837-1923) studied these forces and they are often referred to by his name.We will focus on the three main types:Dispersion (London) forcesDipole-dipole forcesHydrogen bonding

Intermolecular forcesDispersion (London) forcesThe constant vibration of electrons in covalent bonds generates temporary regions of charge distribution and polar regions which result in weak forces of attraction between all covalent molecules.The overall strength of these forces is proportional to;The number of electrons in the moleculeThe size and shape of the molecule

Intermolecular forcesDipole-Dipole ForcesPolar molecules orient themselves so that electrostatic attraction occurs between the different charged regions of the molecules.These forces are generally stronger than the dispersion (London) forces.Their strength depends upon the;Nature (i.e. DE difference) of the polar bondsNumber and placement of the polar bonds Size of the molecules

Intermolecular forcesHydrogen bondingA strong form of dipole-dipole attraction between a hydrogen atom and oxygen, nitrogen or fluorine in a polar covalent molecule.The high electronegative difference results in a strong polar bond and distinct regions of charge distribution.Hydrogen bonding is responsible for waters unique characteristics and influences organic compounds solubility in water.

Assessing intermolecular forcesCan molecules form hydrogen bonds?If so:They have higher boiling and melting points than similar molecules that cannot form hydrogen bondsThey form hydrogen bonds with water and thereby have greater solubility in water

Assessing intermolecular forcesAre they polar?Polar molecules tend to have higher boiling & melting points than similar non-polar ones. Hydrogen bonding capability increases the trend further.A large non-polar region (hydrocarbon) in a molecule reduces the affect of the smaller polar regions and are less soluble than smaller hydrocarbon polar molecules.

Assessing intermolecular forcesHow strong are the dispersion forces?Large hydrocarbon chains have stronger dispersion forces.The boiling and melting points tend to be proportional to the number of carbons in the hydrocarbon.Large hydrocarbons have high boiling and melting points.Small hydrocarbons tend to be gases at SATP.

Hydrocarbons (CnH2n+2) Physical Properties

Polarity of functional groupThe C-H bond is non-polar so most hydrocarbons are non-polarThe addition of halides results in polar regions.The predominant intermolecular force in dispersion forcesHydrogen bondingNoneSolubility in WaterVery poor solubility in water due to the lack of polar and hydrogen bondsMelting & Boiling PointsThe melting and boiling points tend to be very lowThe boiling points increase as the length of the parent chain increases1-5 Cs gases @ SATP5-20 Cs liquids (some very viscous) @SATP20 + Cs solids @ SATP

Hydrocarbons (CnH2n+2)Additional CharacteristicsAs the number of carbons in the parent chain increase:The state of matter at SATP moves from gas to liquid to solidThe melting and boiling points increaseThe density increasesPopular fuel source (Methane, propane, octane, etc.)Source for plastics

Alcohols (R-OH)Physical Properties

Polarity of functional groupOH is very polarAs the parent chain grows, its non-polar nature become more influentialSmall alcohols are more polar than larger onesHydrogen bondingAlcohols experience hydrogen bonding with other alcohols and waterSolubility in WaterThe capacity to form hydrogen bonds make alcohols extremely soluble in water.The solubility decreases as the number of carbons increaseMelting & Boiling PointsDue to the strength of hydrogen bonds most alcohols have higher melting & boiling points than similar alkanes.Most alcohols are liquids at SATP

AlcoholsAdditional CharacteristicsAlcohols are extremely flammableMost alcohols are poisonousMethanol can cause blindness or death.Ethanol is consumed in moderate quantities. Excessive consumption may cause blindness or death. Yeast ferment sugars to produce ethanol and carbon dioxideAntifreeze, cosmetics, foods, medical preparations, solvents, antiseptics, etc.Biological compounds cholesterol, retinol, etc.

Ethers (R-O-R2)Physical Properties

Polarity of functional groupThe C-O bond is less polar than the O-HEthers are less polar than alcoholsThe bent shape around the O aids in the dipole-dipole force strengthHydrogen bondingThere is no hydrogen bonding in ethersEthers can accept hydrogen bonds from waterSolubility in WaterEthers are usually soluble in waterThe solubility decreases as the size of the alkyl groups growsMelting & Boiling PointsMuch lower than comparable alcoholsHigher than similar hydrocarbons

EthersAdditional CharacteristicsEthers are extremely flammableHave been used as anaestheticsether ethoxyethaneMore recent anaesthetic (1-methoxypropane) has fewer side effects.

Amines (R-NH2)Physical Properties

Polarity of functional groupC-N and N-H bonds are polarAmines are usually polarHydrogen bondingThe presence of one or more N-H bonds allows hydrogen bondingSolubility in WaterLow molecular weight amines are soluble in water (

AminesAdditional CharacteristicsWidely found in nature. They are often toxic. Many medical applications.Low molecular weight amines have a fishy smell while others are responsible for odours associated with decay.Amines act as weak bases. Adding an acid produces a salt. So, add lemon to fish to remove fishy odour.

Aldehydes (R-COH) & Ketones Physical Properties

Polarity of functional groupC=O is polarHydrogen bondingNo O-H bonds so no hydrogen bonding between aldehydes and ketonesThe O atom can accept hydrogen bonds from waterSolubility in WaterLow molecular weight aldehydes and ketones have high solubilityThe solubility decreases as the number of carbons increaseMelting & Boiling PointsLower than comparable alcoholsHigher than corresponding alkanes

Aldehydes & KetonesAdditional CharacteristicsAldehydes have a strong pungent odour while ketones smell sweet. As the size of the aldehyde increases the odour becomes more pleasant (i.e. cinnamon) Odours in nature may be used to communicate between insects - pheromonesSince they are polar, they are used as polar solvents. The non-polar alkyl chain permits their use as non-polar solvents too.Antiseptics, disinfectants (Formaldehyde), resins, dyes, preservatives (Aldehyde)Hormones in biology testosterone, progesterone, cortisone Solvents, medication

Carboxylic acids (R-COOH)Physical Properties

Polarity of functional groupPolar due to C=O and O-H bondsHydrogen bondingStrong hydrogen bondingSolubility in WaterLow molecular masses are very soluble (

Carboxylic acidsAdditional CharacteristicsCarboxylic acids often have unpleasant odours butanoic acid smells like stale sweatSour taste in many foods & plants vinegar, spoiled wine, rhubarb, citrus fruits By-product of anaerobic respiration in muscles lactic acidIndustrial processes dyes, tanning, rubber recycling, etc. Food preservatives, Vitamin C, ASAThe OH does not behave like a base. The two Os have high electronegativities and carry the Hs electron allowing the H ion to dissociate.

Boiling & Melting PointsCH3CH2CH2CH3CH3CH2CH2CH2OHAlkaneAldehydeKetoneAlcoholCarboxylic acidb.p. 165.5 oC m.p. -4.5 oCb.p. 117.2 oC m.p. -89.5 oCb.p. 79.6 oC m.p. -86.3 oCb.p. 75.7 oC m.p. -99 oCb.p. -0.5 oC m.p. -138.4 oCLowHigh

Esters (R-COO-R2)Physical Properties

Polarity of functional groupUsually polar due to C=O and C-O bondsPolarity is reduced with increased number of C atomsHydrogen bondingNo O-H bonds, there fore they dont form hydrogen bonds with other estersSolubility in WaterThe C=O bond can accept hydrogen bonds from water, therefore low molecular mass esters are solubleEsters with chains greater than 3 or 4 are not solubleMelting & Boiling PointsThe melting and boiling points for esters are low due to the lack of hydrogen bonding.They are usually volatile liquids at SATP

EstersAdditional CharacteristicsEsters often have pleasant odoursLow molecular mass esters have very pleasant odours and are used in soaps, perfumes, cosmetics, air fresheners, etc.Natural esters are responsible for the scents in flowers & fruits. (Pg 64 Table 2)Food lipids and dietary fats are esters (triglycerides)

Amides (R-CONHR2)Physical Properties

Polarity of functional groupPolar due to C=O,C-N and N-H bondsSimilar physical properties to carboxylic acidsHydrogen bondingSince primary amides have two N-H bonds they have stronger hydrogen bonding than carboxylic acidsSecondary amides also experience hydrogen bondingSolubility in WaterAmides are soluble in waterThe solubility decreases as the non-polar alkyl chain increases in sizeMelting & Boiling PointsPrimary amides have much higher melting and boiling points than carboxylic acidsMany primary amides are solids at SATP

AmidesAdditional CharacteristicsAmides such as acetaminophen are analgesics (pain killers)Urea is a common amide found in urine and fertilizers

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