Properties of organic compounds and functional groups

Carbon has atomic number $Z = 6$: its atom has $6$ electrons, $2$ in the first shell and $4$ in the valence shell. The figure shows these electrons arranged by shells.

1. a) How many electrons is carbon missing to complete its valence shell with $8$?
2. b) Using that fact, explain why each carbon atom forms $4$ covalent bonds.
3. c) Hydrogen has a single electron and its shell is completed with $2$: how many bonds does each hydrogen atom form? Check this by counting bonds in the methane molecule, $\text{CH}_4$.

Carbon has an almost unique property, catenation: its atoms bond to each other to form stable chains. Pentane has its skeleton of $5$ carbons in a straight chain; dimethylpropane has the same $5$ carbons in a branched chain, and cyclopentane, in a closed ring of $5$.

1. a) Draw the carbon skeleton (only the $\text{C-C}$ bonds) of the three molecules in your exercise book.
2. b) Using catenation and these three possibilities (straight-chain, branched and cyclic), explain why millions of different organic compounds are known, whereas hydrogen only forms simple molecules such as $\text{H}_2$ or $\text{H}_2\text{O}$.

You can do it on paper and send a photo with your answer.

For centuries it was believed that organic compounds could only be produced by living organisms thanks to a "vital force". In 1828, Friedrich Wöhler heated ammonium cyanate, an inorganic salt, and obtained urea, $\text{CO(NH}_2)_2$, an organic compound present in urine.

1. a) Explain why that experiment showed that organic compounds can be synthesised in the laboratory without the involvement of any living organism.
2. b) Explain what the word "organic" means in chemistry today: does it have anything to do with whether or not a substance comes from living organisms?
3. c) Diamond and graphite are made solely of carbon: explain whether they are considered organic compounds, taking into account that they are simple substances of a single element and not compounds.

In propane, $\text{C}_3\text{H}_8$, each carbon forms $4$ bonds and each hydrogen, $1$.

1. a) Draw its displayed formula in your exercise book and count the $\text{C-C}$ bonds and the $\text{C-H}$ bonds.
2. b) Check that your counts obey these rules: in a straight-chain alkane of $n$ carbon atoms there are $n - 1$ $\text{C-C}$ bonds and $2n + 2$ $\text{C-H}$ bonds.
3. c) Without drawing it, calculate how many $\text{C-C}$ bonds and how many $\text{C-H}$ bonds hexane, $\text{C}_6\text{H}_{14}$, has.

You can do it on paper and send a photo with your answer.

The four bonds of carbon are oriented in space towards the vertices of a tetrahedron, with angles of $109.5^\circ$: that is the geometry of methane. Around a double bond $\text{C=C}$, as in ethene, the geometry is trigonal planar (angles of $120^\circ$), and around a triple bond, as in ethyne, linear ($180^\circ$).

1. a) Draw the three molecules in your exercise book, indicating the type of carbon-carbon bond and the geometry of each one.
2. b) Order single, double and triple bonds from longest to shortest, assigning these length values: $154\ \text{pm}$, $134\ \text{pm}$ and $120\ \text{pm}$.
3. c) Also assign to each bond its bond dissociation energy ($348$, $612$ and $838\ \text{kJ/mol}$) and state the conclusion that relates length, energy and number of shared electron pairs.

You can do it on paper and send a photo with your answer.

Silicon is directly below carbon in the periodic table and also forms $4$ bonds, but there is no 'silicon chemistry' comparable to carbon chemistry. Bond energies, in $\text{kJ/mol}$: $\text{C-C} = 348$; $\text{Si-Si} = 226$; $\text{C-O} = 358$; $\text{Si-O} = 452$. The figure compares the four energies.

1. a) Compare the energies of the $\text{C-C}$ and $\text{Si-Si}$ bonds and explain why long chains of silicon atoms break easily.
2. b) Compare the energies of the $\text{C-O}$ and $\text{Si-O}$ bonds and reason why silicon always occurs in nature bonded to oxygen (sand, silicates) rather than forming its own chains.
3. c) Silicones used in sealants and baking moulds are polymers with a $-\text{Si-O-Si-O}-$ backbone: relate this to the previous data.

The label of an isotonic drink and the laboratory cupboard contain these substances: glucose ($\text{C}_6\text{H}_{12}\text{O}_6$), sodium chloride ($\text{NaCl}$), water ($\text{H}_2\text{O}$), ethanol ($\text{C}_2\text{H}_6\text{O}$), methane ($\text{CH}_4$) and calcium carbonate ($\text{CaCO}_3$).

1. a) Classify them into organic and inorganic compounds.
2. b) State which chemical element all the organic compounds on the list share.
3. c) Calcium carbonate contains that element and yet is studied as an inorganic compound: propose a rule of thumb explaining why some carbon-containing compounds are organic and others (such as $\text{CO}_2$ or carbonates) are not.

In the laboratory, a spoonful of sugar (sucrose) and a spoonful of common salt ($\text{NaCl}$) are compared. Observations: salt melts at $801\ \celsius$ without decomposing, whereas sugar burns and decomposes at around $186\ \celsius$; water with dissolved salt conducts electricity and water with sugar hardly does at all; sugar burns when heated in the presence of air and salt is non-combustible. The figure shows the two structures.

1. a) Relate each observation to the bonding type of each substance: ionic lattice in salt, covalent molecules in sugar.
2. b) State two general properties that distinguish most organic compounds from inorganic salts.

The molecular formula $\text{C}_5\text{H}_{12}$ corresponds to three different substances: pentane, straight-chain (boiling point $36.1\ \celsius$), methylbutane, with one branch ($27.7\ \celsius$), and dimethylpropane, with two ($9.5\ \celsius$).

1. a) Draw the three carbon skeletons in your exercise book and verify that all three have $5$ carbons and $12$ hydrogens.
2. b) What are compounds that share the same molecular formula but have different structures called?
3. c) Order the data and conclude how branching affects the boiling point; propose an explanation by considering how molecules pack together and touch one another.

You can do it on paper and send a photo with your answer.

To determine the composition of a hydrocarbon X, a sample is burned: $0.2\ \text{mol}$ of X consume $0.9\ \text{mol}$ of $\text{O}_2$ and produce $0.6\ \text{mol}$ of $\text{CO}_2$ and $0.6\ \text{mol}$ of $\text{H}_2\text{O}$.

1. a) Calculate how many carbon atoms and how many hydrogen atoms each molecule of X contains: the moles of $\text{CO}_2$ reveal all the carbon, and those of $\text{H}_2\text{O}$, all the hydrogen.
2. b) Check with an oxygen atom balance (those in the products compared with those supplied by $\text{O}_2$) that X contains no oxygen.
3. c) Write down the molecular formula of X and propose two structures compatible with it: a chain with a double bond and a three-carbon ring.

A functional group is an atom or group of atoms that, when present in different molecules, gives them common characteristic properties. Methanol, $\text{CH}_3\text{OH}$, ethanol, $\text{CH}_3\text{CH}_2\text{OH}$, and $1$-propanol, $\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}$, share the $-\text{OH}$ (hydroxyl) group.

1. a) Which family do the three compounds belong to?
2. b) Separate in each formula which part is a carbon-hydrogen chain and which part is the functional group.
3. c) State a common property of these substances due to the $-\text{OH}$ group (think, for example, about their behaviour with water).

Each of these everyday substances contains a different functional group: methanoic acid from ant stings, $\text{HCOOH}$; ethanal from green apple aroma, $\text{CH}_3\text{CHO}$; propanone from nail polish remover, $\text{CH}_3\text{COCH}_3$; and methylamine, $\text{CH}_3\text{NH}_2$, responsible for the smell of stale fish.

1. a) Locate the functional group in each formula: carboxyl ($-\text{COOH}$), carbonyl ($\text{C=O}$) or amino ($-\text{NH}_2$).
2. b) Assign each compound to its family: carboxylic acid, aldehyde, ketone or amine.
3. c) Distinguish between the two carbonyl-containing compounds: how does an aldehyde differ from a ketone regarding the position of the group?

Ethyl ethanoate, $\text{CH}_3\text{COOCH}_2\text{CH}_3$, is responsible for the aroma of ripe pears, and dimethyl ether, $\text{CH}_3\text{OCH}_3$, is used as a propellant gas in some aerosols.

1. a) Locate the functional group in each formula: ester ($-\text{COO}-$) or ether (an $-\text{O}-$ joining two chains).
2. b) Explain the structural difference between both groups: what extra atom accompanies the oxygen in the ester group?
3. c) Assign each compound to its family.

Methanal (formaldehyde), $\text{HCHO}$, is a gas that, dissolved in water at $40\ \%$ by mass ("formalin"), is used as a preservative for biological specimens.

1. a) Identify its functional group and state which family it belongs to.
2. b) Draw its displayed formula in your exercise book: count the $\text{C-H}$ bonds and the $\text{C=O}$ bond, and explain why it is the smallest possible aldehyde.
3. c) Industrially, it is obtained by removing two hydrogen atoms from methanol: $\text{CH}_3\text{OH} \rightarrow \text{HCHO} + \text{H}_2$. Check the atom balance and explain why removing hydrogen from an organic compound is equivalent to oxidising it.

You can do it on paper and send a photo with your answer.

Butanal, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}$, is obtained by gently oxidising $1$-butanol, and propanone, $\text{CH}_3\text{COCH}_3$, by oxidising $2$-propanol.

1. a) Locate the carbonyl group, $\text{C=O}$, in each product and explain the difference in position: at one end of the chain or on an internal carbon.
2. b) Assign each product to its family: aldehyde or ketone.
3. c) Deduce and write down the general rule: which type of alcohol (with the $-\text{OH}$ at one end or on an internal carbon) gives an aldehyde and which gives a ketone.

Hexane, ethanol and ethanoic acid are poured separately into three test tubes containing water and shaken. Note that the $-\text{OH}$ and $-\text{COOH}$ groups are polar and form hydrogen bonds with water, whereas a carbon-hydrogen chain is non-polar.

1. a) Predict which liquids will be miscible with water and which one will form two layers, like oil.
2. b) State the rule "like dissolves like" and apply it to the result.
3. c) Explain why a little white spirit (used with gloves, good ventilation and away from flames) works better than water to clean a grease stain from a bicycle chain, and why alcohol rubs off permanent marker ink better.

To descale limestone deposits from the bottom of a coffee machine, vinegar is used: its ethanoic acid reacts with the carbonate and dissolves it, releasing $\text{CO}_2$. In the laboratory, the acidity of carboxylic acids is tested by adding sodium bicarbonate:

$\text{CH}_3\text{COOH} + \text{NaHCO}_3 \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$

1. a) Check that the equation is balanced by counting the atoms of each element on both sides.
2. b) Identify the functional group of ethanoic acid.
3. c) Explain what general property of carboxylic acids this test demonstrates and what you would observe when carrying it out.

Methylamine, $\text{CH}_3\text{NH}_2$, is a foul-smelling gas related to ammonia, $\text{NH}_3$: both have basic character because their nitrogen atom can gain a $\text{H}^+$ from water, leaving $\text{OH}^-$ ions in the solution.

1. a) Explain what it means for a substance to have a basic character and reason whether the pH of water will rise or fall when dissolving methylamine.
2. b) Identify the functional group of methylamine and the family to which it belongs.
3. c) Glycine, an amino acid, has the condensed structural formula $\text{H}_2\text{N-CH}_2\text{-COOH}$: identify its two functional groups and the two families to which it belongs simultaneously.

In the laboratory, there are three unlabelled bottles containing ethanol, ethanoic acid, and ethyl ethanoate, without knowing which is which. You only have pH indicator paper, sodium bicarbonate, and your sense of smell (applying the correct technique, without directly inhaling).

1. a) Explain what you would observe when applying each test to each substance: acidic or neutral pH, fizzing or no fizzing with bicarbonate, fruity odour or not.
2. b) Design an order of testing that allows you to identify the three bottles using the fewest possible tests, and justify it using the functional groups of each compound.

Stearic acid, $\text{C}_{17}\text{H}_{35}\text{-COOH}$, is a very long-chain carboxylic acid present in fats, and its sodium salt is the main ingredient in many bars of soap. The figure schematically represents its molecule.

1. a) Predict whether stearic acid will be soluble or insoluble in water and explain the paradox: it carries a polar $-\text{COOH}$ group, which by itself would be soluble, attached to a non-polar $17$ carbon chain.
2. b) Describe how its molecules orient themselves on the surface of water and around a droplet of grease: which end touches the water and which end touches the grease.
3. c) Using this picture, explain why soap enables grease to be washed away with water.

A poorly sealed bottle of wine turns to vinegar: oxygen from the air oxidises ethanol, converting it into ethanoic acid, according to the equation $\text{CH}_3\text{CH}_2\text{OH} + \text{O}_2 \rightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O}$.

1. a) Identify the functional group of the organic reactant and that of the product, and explain what the transformation consists of from the point of view of the atoms: the molecule 'gains' an oxygen atom.
2. b) Calculate the mass of ethanoic acid obtained when $46\ \text{g}$ of ethanol is oxidised and the mass of $\text{O}_2$ consumed (molar masses: ethanol $= 46$; ethanoic acid $= 60$; $\text{O}_2 = 32\ \text{g/mol}$).
3. c) Calculate the percentage increase in mass of the organic substance when changing from alcohol to acid.

Two substances, A and B, have the same molecular formula, $\text{C}_2\text{H}_6\text{O}$. At room temperature, A is a liquid that boils at $78.4\ \celsius$ and is miscible with water in all proportions; B is a gas that liquefies at $-24\ \celsius$ and barely dissolves in water.

1. a) Assign the substances: one is ethanol and the other is dimethyl ether.
2. b) Write down the condensed structural formulas for both and point out their functional groups.
3. c) Justify the huge difference in boiling points and solubility: only one of the molecules has an $\text{O-H}$ bond capable of forming hydrogen bonds.
4. d) A and B are said to be functional group isomers, having the same molecular formula but different functional groups: define this concept using this example.

Glycine, $\text{H}_2\text{N-CH}_2\text{-COOH}$, is the simplest amino acid. In water, its $-\text{COOH}$ group gives up a $\text{H}^+$, like any carboxylic acid, and its $-\text{NH}_2$ group accepts a $\text{H}^+$, like any amine, so that the molecule is left with two charges simultaneously: $\text{H}_3\text{N}^+\text{-CH}_2\text{-COO}^-$.

1. a) Identify the two functional groups of glycine and classify it into two families at the same time.
2. b) Explain what it means for glycine to be an amphoteric substance: how will it react with a strong acid? And with a strong base?
3. c) Proteins in the body are long chains of amino acids: reason why they can act as 'sponges' for $\text{H}^+$ ions and help keep blood pH stable.